Coil Actuated Position Sensor With Reflected Magnetic Field

ABSTRACT

A magnetic field sensor includes at least one coil responsive to an AC coil drive signal; at least two spaced apart magnetic field sensing elements responsive to a sensing element drive signal and positioned proximate to the at least one coil; and a circuit coupled to the at least two magnetic field sensing elements to generate an output signal of the magnetic field sensor indicative of a difference between a distance of a conductive target with respect to each of the at least two spaced apart magnetic field sensing elements.

FIELD

This application relates to magnetic field detection and, morespecifically, to detection of a reflected field from a magnetic target

BACKGROUND

Magnetic field sensors are often used to detect a ferromagnetic target.They often act as sensors to detect motion or position of the target.Such sensors are ubiquitous in many areas of technology includingrobotics, automotive, manufacturing, etc. For example, a magnetic fieldsensor may be used to detect when a vehicle's wheel locks up, triggeringthe vehicle's control processor to engage the anti-lock braking system.In this example, the magnetic field sensor may detect rotation of thewheel. Magnetic field sensor may also detect distance to an object. Forexample, a magnetic field sensor may be used to detect the position of ahydraulic piston.

SUMMARY

In an embodiment, a magnetic field sensor includes at least one coilresponsive to an AC coil drive signal; at least two spaced apartmagnetic field sensing elements responsive to a sensing element drivesignal and positioned proximate to the at least one coil; and a circuitcoupled to the at least two magnetic field sensing elements to generatean output signal of the magnetic field sensor indicative of a differencebetween a distance of a conductive target with respect to each of the atleast two spaced apart magnetic field sensing elements.

One or more of the following features may be included.

At least one of the magnetic field sensing elements may be configured todetect a directly coupled magnetic field generated by the at least onecoil and a reflected magnetic field reflected by the conductive target,wherein a configuration of the at least one coil and the at least twomagnetic field sensing elements is selected in order to achieve apredetermined level for the directly coupled magnetic field.

The configuration of the at least one coil and the at least two magneticfield sensing elements may be selected to minimize the predeterminedlevel of the directly coupled magnetic field.

The at least one coil may comprise a pair of coils and wherein the atleast two magnetic field sensing elements are disposed between thecoils.

The at least one coil may comprise a single coil.

The at least one coil may comprise a first portion configured to have acurrent flow in a first direction and a second portion configured tohave a current flow in a second direction, wherein at least one of themagnetic field sensing elements is disposed above the first portion ofthe at least one coil and at least one of the magnetic field sensingelements is disposed above the second portion of the at least one coil.

At least one of the magnetic field sensing elements may be aligned witha gap of the at least one coil.

At least one coil may comprise a first plurality of loops and a secondplurality of loops spaced from the first plurality of loops by a gap andwherein at least one of the magnetic field sensing elements is alignedwith the gap.

At least one coil may comprise at least two layers, wherein at least oneof the magnetic field sensing elements is disposed between the at leasttwo layers of the at least one coil.

The magnetic field sensing elements may include at least four magneticfield sensing elements electrically coupled to form at least one bridge.

The magnetic field sensing elements may comprise at least eight magneticfield sensing elements electrically coupled to form at least twobridges.

The conductive target may comprise a pressure susceptible element andthe output signal is further indicative of a pressure associated withthe pressure susceptible element.

The AC coil drive signal may include a first frequency and the sensingelement drive signal may include a second frequency.

The first frequency of the AC coil drive signal may be selected based atleast in part on a skin effect associated with the conductive target.

The AC coil drive signal and the sensing element drive signal may beprovided by a common source, wherein the first frequency of the AC coildrive signal and the second frequency of the sensing element drivesignal are substantially the same.

The first frequency and second frequency may be selected in response tochanging the first frequency from a first frequency value to a secondfrequency value and comparing the output signal associated with thefirst frequency value to the output signal associated with the secondfrequency value to detect a disturbance.

The AC coil drive signal and the sensing element drive signal may beprovided by independent sources.

The second frequency of the sensing element drive signal may beapproximately DC.

The first frequency of the AC coil drive signal may be selected inresponse to changing of the first frequency from a first frequency valueto a second frequency value and comparing the output signal associatedwith the first frequency value to the output signal associated with thesecond frequency value to detect a disturbance.

The circuit may comprise a low pass filter.

The circuit may comprise a temperature compensator responsive to atemperature sensor and to a material type selector.

The circuit may comprise a linearization module.

The AC coil drive signal may have a first phase, wherein each of the atleast two magnetic field sensing elements generates a respectivemagnetic field signal, and wherein the magnetic field sensor furthercomprises at least one feedback coil driven by a second AC coil drivesignal having a second phase and disposed adjacent to the at least twomagnetic field sensing elements, wherein the circuit is configured toadjust the second AC coil drive signal in order to achieve apredetermined level for the magnetic field signals generated by the atleast two magnetic field sensing elements.

A current detection circuit may be included to detect the second AC coildrive signal and to provide the output signal of the magnetic fieldsensor based on the detected second AC coil drive signal.

The AC coil drive signal may have a first phase and at least one of themagnetic field sensing elements may detect a reflected magnetic fieldreflected by the conductive target and having a second phase, whereinthe circuit comprises a demodulator responsive to a difference betweenthe first phase and the second phase to demodulate the magnetic fieldsignal.

The magnetic field sensing elements may comprise one or more of a Halleffect element, a giant magnetoresistance (MR) element, an anisotropicmagnetoresistance (AMR) element, a tunneling magnetoresistance (TMR)element, or a magnetic tunnel junction (MTJ) element.

In another embodiment, a magnetic field sensor includes a substrate; atleast one coil supported by the substrate and responsive to an AC coildrive signal; a first set of at least two magnetic field sensingelements disposed at a first position with respect to the coil; a secondset of at least two magnetic field sensing elements disposed at a secondposition with respect to the coil, the second position spaced from thefirst position, wherein the second set of magnetic field sensingelements is electrically coupled to the first set of magnetic fieldsensing elements to form a bridge; and a circuit coupled to the bridgeto generate an output signal indicative of a difference between a firstdistance of a conductive target with respect to the first set ofmagnetic field sensing elements and a second distance of the conductivetarget with respect to the second set of magnetic field sensingelements.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features may be more fully understood from the followingdescription of the drawings. The drawings aid in explaining andunderstanding the disclosed technology. Since it is often impractical orimpossible to illustrate and describe every possible embodiment, theprovided figures depict one or more examples of embodiments.Accordingly, the figures are not intended to limit the scope of theinvention. Like numbers in the figures denote like elements.

FIG. 1 is a block diagram of a system for sensing a target.

FIG. 2 is an isometric diagram of a system for sensing a target.

FIG. 2A shows cross-sectional views of the system of FIG. 2.

FIG. 3 is a schematic diagram of a coil and magnetoresistance (MR)elements for sensing a target.

FIG. 3A is a schematic diagram of an embodiment of a coil and MRelements for sensing a target, including bond pads.

FIG. 3B is schematic diagram of an embodiment of coil and MR elementsfor sensing a target.

FIG. 4 is a cross-sectional view of a system for sensing a target.

FIG. 5 is a schematic diagram of a coil and MR elements for sensing atarget.

FIG. 5A is schematic diagram of an embodiment of a coil and MR elementsfor sensing a target.

FIG. 5B is schematic diagram of an embodiment of a coil and MR elementsfor sensing a target, including a lead frame.

FIG. 5C is schematic diagram of an embodiment of a coil and MR elementsfor sensing a target.

FIG. 6 is schematic diagram of an embodiment of a coil and MR elementsfor sensing a target.

FIG. 7 is a cross-sectional view of coils and MR elements for sensing atarget.

FIG. 8 is an isometric view of a pressure sensor.

FIG. 8A is an isometric view of embodiments of the pressure sensor ofFIG. 8.

FIG. 9 is a cross-sectional view of an embodiment of a pressure sensorincluding substrates.

FIG. 10 is a block diagram of a circuit for sensing a magnetic target.

FIG. 10A is a block diagram of an embodiment of a circuit for sensing amagnetic target.

FIG. 11 is a block diagram of an embodiment of a circuit for sensing amagnetic target.

FIG. 11A is a block diagram of an embodiment of a circuit for sensing amagnetic target.

FIG. 11B is a block diagram of an embodiment of a circuit for sensing amagnetic target.

FIG. 11C is a block diagram of an embodiment of a circuit for sensing amagnetic target.

FIG. 11D is a block diagram of an embodiment of a circuit for sensing amagnetic target.

FIG. 11E is a block diagram of an embodiment of a circuit for sensing amagnetic target.

FIG. 11F is a block diagram of an embodiment of a circuit for sensing amagnetic target.

FIG. 12 is a diagram representing an output signal for a system withsensitivity detection.

FIG. 12A is a block diagram of a magnetic field detection circuit withsensitivity detection.

FIG. 12B is a block diagram of an embodiment of a magnetic fielddetection circuit with sensitivity detection.

FIG. 12C is a block diagram of an embodiment of a magnetic fielddetection circuit with sensitivity detection.

FIG. 13 is a schematic diagram of an embodiment of a magnetic fielddetection circuit with sensitivity detection including a coil and MRelements.

FIG. 13A is a schematic diagram of an embodiment of a coil havingcountercoils and gaps between traces.

FIG. 13B is a block diagram of an embodiment of a magnetic fielddetection circuit with sensitivity detection.

FIG. 14 is a side view of a magnetic field sensor and a magnetic targethaving material of varying thickness.

FIG. 14A is a side view of a magnetic field sensor and a magnetic targethaving material of varying thickness.

FIG. 14B is a side view of a magnetic field sensor and a magnetic targethaving material of varying thickness.

FIG. 15 is a side view of a magnetic field sensor and a magnetic targethaving material with multiple thicknesses.

FIG. 15A is a side view of a magnetic field sensor and a magnetic targethaving material with multiple thicknesses.

FIG. 15B is a side view of a magnetic field sensor and a magnetic targethaving material with multiple thicknesses.

FIG. 15C is a side view of a magnetic field sensor and a magnetic targethaving material with multiple thicknesses.

FIG. 16 is a side view of a magnetic field sensor and a magnetic targethaving an inclined plane.

FIG. 16A is a side view of a magnetic field sensor and a magnetic targethaving an inclined plane.

FIG. 17 is a side view of a substrate and lead frame connected by leadwires.

FIG. 17A is a side view of a substrate and lead frame connected bysolder bumps.

FIG. 18 is a schematic diagram of a dual-die package including one ormore coils.

FIG. 18A is a schematic diagram of a dual-die package including one ormore coils.

FIG. 19 is a schematic diagram of a multi-die package including one ormore coils.

DETAILED DESCRIPTION

As used herein, the term “magnetic field sensing element” is used todescribe a variety of electronic elements that can sense a magneticfield. The magnetic field sensing element can be, but is not limited to,a Hall Effect element, a magnetoresistance (MR) element, or amagnetotransistor. As is known, there are different types of Hall Effectelements, for example, a planar Hall element, a vertical Hall element,and a Circular Vertical Hall (CVH) element. As is also known, there aredifferent types of magnetoresistance elements, for example, asemiconductor magnetoresistance element such as Indium Antimonide(InSb), a giant magnetoresistance (MR) element, an anisotropicmagnetoresistance element (AMR), a tunneling magnetoresistance (TMR)element, and a magnetic tunnel junction (MTJ). The magnetic fieldsensing element may be a single element or, alternatively, may includetwo or more magnetic field sensing elements arranged in variousconfigurations, e.g., a half bridge or full (Wheatstone) bridge.Depending on the device type and other application requirements, themagnetic field sensing element may be a device made of a type IVsemiconductor material such as Silicon (Si) or Germanium (Ge), or a typeIII-V semiconductor material like Gallium-Arsenide (GaAs) or an Indiumcompound, e.g., Indium-Antimonide (InSb).

As is known, some of the above-described magnetic field sensing elementstend to have an axis of maximum sensitivity parallel to a substrate thatsupports the magnetic field sensing element, and others of theabove-described magnetic field sensing elements tend to have an axis ofmaximum sensitivity perpendicular to a substrate that supports themagnetic field sensing element. In particular, planar Hall elements tendto have axes of sensitivity perpendicular to a substrate, while metalbased or metallic magnetoresistance elements (e.g., MR, TMR, AMR) andvertical Hall elements tend to have axes of sensitivity parallel to asubstrate.

As used herein, the term “magnetic field sensor” is used to describe acircuit that uses a magnetic field sensing element, generally incombination with other circuits. Magnetic field sensors are used in avariety of applications, including, but not limited to, an angle sensorthat senses an angle of a direction of a magnetic field, a currentsensor that senses a magnetic field generated by a current carried by acurrent-carrying conductor, a magnetic switch that senses the proximityof a ferromagnetic object, a rotation detector that senses passingferromagnetic articles, for example, magnetic domains of a ring magnetor a ferromagnetic target (e.g., gear teeth) where the magnetic fieldsensor is used in combination with a back-biased or other magnet, and amagnetic field sensor that senses a magnetic field density of a magneticfield.

As used herein, the terms “target” and “magnetic target” are used todescribe an object to be sensed or detected by a magnetic field sensoror magnetic field sensing element. The target may comprise a conductivematerial that allows for eddy currents to flow within the target, forexample a metallic target that conducts electricity.

FIG. 1 is a block diagram of a system 100 for detecting a conductivetarget 102. Target 102 may be magnetic or non-magnetic in variousembodiments. System 100 includes one or more magnetoresistance (MR)elements 104 and an MR driver circuit 106. MR driver circuit may includea power supply or other circuit that provides power to MR elements 104.In embodiments, MR elements 104 may be replaced with other types ofmagnetic field sensing elements such as Hall effect elements, etc. MRelements 104 may comprise a single MR element or multiple MR elements.The MR elements may be arranged in a bridge configuration, in certainembodiments.

System 100 may also include one or more coils 108 and a coil drivercircuit 110. Coils 108 may be electrical coils, windings, wires, traces,etc. configured to generate a magnetic field when current flows throughthe coils 108. In embodiments, coils 108 comprise two or more coils,each a conductive trace supported by substrate, such as a semiconductorsubstrate, a glass substrate, a ceramic substrate, or the like. In otherembodiments, coils 108 may not be supported by a substrate. For example,coils 108 may be supported by a chip package, a frame, a PCB, or anyother type of structure that can support traces of a coil. In otherembodiments, coils 108 may be free standing wire, i.e. not supported bya separate supporting structure.

Coil driver 110 is a power circuit that supplies current to coils 108 togenerate the magnetic field. In an embodiment, coil driver 110 mayproduce an alternating current so that coils 108 produce alternatingmagnetic fields (i.e. magnetic fields with magnetic moments that changeover time). Coil driver 110 may be a circuit implemented, in whole or inpart, on the semiconductor die.

System 100 may also include processor 112 coupled to receive signal 104a from MR elements 104, which may represent the magnetic field asdetected by MR elements 104. Processor 100 may receive signal 104 a anduse it to determine a position, speed, direction, or other property oftarget 102.

MR elements 104 and coils 108 may be positioned on substrate 114.Substrate 114 may comprise semiconductor substrates, such as siliconsubstrates, a chip package, PCB or other type of board-level substrates,or any type of platform that can support MR elements 104 and coils 108.Substrate 114 may include a single substrate or multiple substrates, aswell as a single type of substrate or multiple types of substrates.

In operation, MR driver 106 provides power to MR elements 104 and coildriver 110 provides current to coils 108. In response, coils 108 producea magnetic field that can be detected by MR elements 104, which producesignal 104 a representing the detected magnetic field.

As target 102 moves in relation to the magnetic field, its position andmovement through the field changes the field. In response, signal 104 aproduced by MR elements 104 changes. Processor 112 receives signal 104 aand processes the changes in (and/or the state of) the signal todetermine position, movement, or other characteristics of target 102. Inan embodiment, system 100 can detect movement or position of target 102along axis 116. In other words, system 100 may detect the position oftarget 102 in proximity to MR elements 104 as target 102 moves toward oraway from MR elements 104 and coils 108. System 102 may also be able todetect other types of position or movement of target 102.

Referring now to FIG. 2, system 200 may be the same as or similar tosystem 100. Substrate 202 may be the same as or similar to substrate114, and may support coil 204, coil 206, and MR element 208. Althoughone MR element is shown, MR element 208 may comprise two or more MRelements depending on the embodiment of system 200. Target 203 may bethe same as or similar to target 102.

Although not shown, an MR driver circuit 106 may provide current to MRelement 208 and coil driver circuit 110 may provide current to coils 204and 206.

Coil 204 and 206 may be arranged so that the current flows through coils204 and 206 in opposite directions, as shown by arrow 208 (indicating aclockwise current in coil 204) and arrow 210 (indicating acounterclockwise current in coil 206). As a result, coil 204 may producea magnetic field having a magnetic moment in the negative Z direction(i.e. down, in FIG. 2), as indicated by arrow 212. Similarly, coil 206may produce a magnetic field having a magnetic moment in the oppositedirection, the positive Z direction, as indicated by arrow 214. Anaggregate magnetic field 211 produced by both coils may have a shapesimilar to that shown by magnetic field lines 211. It will beappreciated that coils 204, 206 may be formed by a single coil structurerespectively wound so that the current through the coils flows inopposite directions. Alternatively, coils 204, 206 may be formed byseparate coil structures.

In an embodiment, MR element 208 may be placed between coils 204 and206. In this arrangement, absent any other magnetic fields aside fromthose produced by coils 204 and 206, the net magnetic field at MRelement 208 may be zero. For example, the negative Z component of themagnetic field produced by coil 204 may be canceled out by the positiveZ component of the magnetic field produced by coil 206, and the negativeX component of the magnetic field shown above substrate 202 may becanceled out by the positive X component of the magnetic field shownbelow substrate 202. In other embodiments, additional coils may be addedto substrate 202 and arranged so that the net magnetic field at MRelement 208 is substantially nil.

To achieve a substantially zero magnetic field at the location of MRelement 208, coil 204 and coil 206 may be placed so that current throughthe coils flows in circular patterns substantially in the same plane.For example, the current through coil 204 and 206 is flowing in circularpatterns through the coils. As shown, those circular patterns aresubstantially coplanar with each other, and with the top surface 216 ofsubstrate 202.

As noted above, coil driver 110 may produce an alternating field. Inthis arrangement, the magnetic field shown by magnetic field lines 211may change direction and magnitude over time. However, during thesechanges, the magnetic field at the location of MR element 208 may remainsubstantially nil.

In operation, as target 203 moves toward and away from MR element 208(i.e. in the positive and negative Z direction), magnetic field 211 willcause eddy currents to flow within target 203. These eddy currents willcreate their own magnetic fields, which will produce a non-zero magneticfield in the plane of the MR element 208, which non-zero magnetic fieldcan be sensed to detect the motion or position of target 203.

Referring to FIG. 2A, a cross-sectional view 250 of system 200, asviewed at line 218 in the Y direction, illustrates the eddy currentswithin target 203. The ‘x’ symbol represents a current flowing into thepage and the symbol represents a current flowing out of the page. Asnoted above, the current through coils 204 and 206 may be an alternatingcurrent, which may result in an alternating strength of magnetic field211. In embodiments, the phase of the alternating current through coil204 matches the phase of the alternating current through coil 206 sothat magnetic field 211 is an alternating or periodic field.

Alternating magnetic field 211 may produce reflected eddy currents 240and 242 within magnetic target 203. Eddy currents 240 and 242 may beopposite in direction to the current flowing through coils 204 and 206,respectively. As shown, eddy current 246 flows out of the page and eddycurrent 248 flows into the page, while coil current 251 flows into thepage and current 252 flows out of the page. Also, as shown, thedirection of eddy current 242 is opposite the direction of the currentthrough coil 206.

Eddy currents 240 and 242 form a reflected magnetic field 254 that has adirection opposite to magnetic field 211. As noted above, MR element 208detects a net magnetic field of zero due to magnetic field 211. However,MR element 208 will detect a non-zero magnetic field in the presence ofreflected magnetic field 254. As illustrated by magnetic field line 256,the value of reflected magnetic field 254 is non-zero at MR element 208.

As target 203 moves closer to coils 204 and 206, magnetic field 211 mayproduce stronger eddy currents in target 203. As a result, the strengthof magnetic field 254 may change. In FIG. 2A, magnetic field 211′ (inthe right-hand panel of FIG. 2A) may represent a stronger magnetic fieldthan magnetic field 211 due, for example, to the closer proximity oftarget 203 to coils 204 and 206. Thus, eddy currents 240′ and 242′ maybe stronger currents than eddy currents 240 and 242, and magnetic field254′ may be stronger than magnetic field 254. This phenomenon may resultin MR element 208 detecting a stronger magnetic field (i.e. magneticfield 254′) when target 203 is closer to coils 204 and 206, and a weakermagnetic field (i.e. magnetic field 254) when target 203 is further awayfrom coils 204 and 206.

Also, eddy currents 240′ and 242′ generally occur on or near the surfaceof target 203. Therefore, as target 203 moves closer to co MR element208, MR element 208 may experience a stronger magnetic field from theeddy currents because the source of the magnetic field is closer to MRelement 208.

FIG. 3 is a schematic diagram of a circuit 300 including coils 302 and304, and MR elements 306 and 308. Coils 302 and 304 may be the same asor similar to coils 204 and 206, and MR elements 306 and 308 may each bethe same as or similar to MR element 208.

In an embodiment, coils 302 and 304, and MR elements 306 and 308 may besupported by a substrate. For example, coils 302 and 304 may compriseconductive traces supported by a substrate and MR elements 306 and 308may be formed on a surface of or in the substrate.

In an embodiment, coils 302 and 304 may comprise a single conductivetrace that carries current. The portion of the trace forming coil 302may loop or spiral in a direction opposite to the portion of the traceforming coil 304, so that the current through each coil is equal andflows in opposite directions. In other embodiments, multiple traces maybe used.

Coils 302 and 304 are symmetrically positioned on opposite sides of MRelements 306 and 308, with MR elements 308 and 304 in the middle. Thismay result in MR elements 306 and 308 being in the center of themagnetic field produced by coils 302 and 304, so that, absent any otherstimulus, the magnetic field detected by MR elements 306 and 308 as aresult of magnetic fields produced by coils 302 and 304 (referred toherein as the directly coupled magnetic field) is substantially nil.

FIG. 3A is a schematic diagram of an embodiment of a magnetic fielddetection circuit 300′, which may be the same as or similar to system100 in FIG. 1. Coils 302 and 304 may be supported by a substrate asdescribed above. Circuit 300′ may include four MR elements 310, 312,314, and 316, which may be coupled in a bridge configuration 318. Inembodiments, bridge 318 may produce a differential output consisting ofsignals 318 a and 318 b.

Arranging the MR elements in a bridge may, in certain embodiments,increase the sensitivity of the magnetic field sensor. In an embodiment,a target is movable with respect to the circuit 300′ such that as thetarget approaches the circuit it mainly moves towards MR elements 310,312, but not towards MR elements 314, 316. With this configuration, theresistance of MR elements 310 and 312 may change and the resistance ofMR elements 314 and 316 may remain relatively constant as the targetapproaches and recedes from the MR elements. If, for example, MRelements are aligned so that the MR resistance of 310, 312 decreases andthe resistance of MR elements 314, 316 increases as the targetapproaches, then signal 318 a will decrease and signal 318 b willincrease in voltage as the target approaches. The opposite reaction ofthe MR elements (and the differential signals 318 a and 318 b) mayincrease sensitivity of the magnetic field detection circuit while alsoallowing the processor that receives the differential signal to ignoreany common mode noise.

In embodiments, arranging MR elements 310-316 in a bridge may allow fordetection of the difference in the position of the target over the setof resistors and/or detection of a phase difference between the bridgeoutputs. This may be utilized, for example, to detect tilt ordeformation of a target.

Circuit 300′ may also include a bond pads 320 having multiple leads 322that can be accessed and form connections external to a chip package(not shown). Lead wires or conductive traces 324 may connect MR elements310, 312, 314, and 316 to external leads or pads 322 so they can becoupled to other circuits like, for example, MR driver 106.

Referring to FIG. 3B, a circuit 323 includes four coils 324-330 andthree rows 332, 334, and 336 of MR elements. Circuit 323 may be used todetect location or motion of a target.

The coils may produce magnetic fields in alternating patterns. Forexample, coil 324 may produce a field going into the page, coil 326 mayproduce a field coming out of the page, coil 328 may produce a fieldgoing into the page, and coil 330 may produce a field coming out of thepage. As a result, the magnetic field detected by the MR elements inrows 332, 334, and 336 as a result of magnetic fields produced by coils324, 326, 328, 330 may be substantially nil.

Circuit 323 may also be extended by adding additional coils andadditional MR elements. In embodiments, the additional coils may beconfigured to create magnetic fields with alternating directions, asdescribed above, and the MR elements between the coils may be placed sothat they detect a magnetic field that is substantially nil.

The MR elements in rows 332, 334, and 336 may form a grid. As a targetmoves above the grid and approaches the MR elements, the MR elementswill be exposed to and detect the reflected magnetic field produced bythe eddy currents flowing in the target as a result of the magneticfields produced by the coils 324-330. For example, if a target movesover MR elements 338 and 340, those MR elements may detect the reflectedmagnetic field and produce an output signal indicating as much. Aprocessor receiving the output signals from the MR elements can thenidentify the location of the target as above or near MR elements 338 and340. If the target then moves close to MR element 342, MR element 342will detect the reflected magnetic field from the target and produce anoutput signal indicating the target was detected. The processorreceiving the output signals can then identify the location of thetarget as above or near MR element 342.

A single large target may be placed in front of the grid 332,334 and336. Then the difference of reflected fields experienced by each MRelement is a mapping of the parallelism of the target and the plane ofthe grid. It can be also used to map the deformations of the target asfunction of an external constraint.

Referring to FIG. 4, a system 400 for detecting a target 402 may use asingle coil and MR element to detect target 402. MR element 404 may beplaced proximate to coil 406. In an embodiment, MR element 404 may beplaced between coil 406 and target 402. In other embodiments, the tracesof coil 406 may be placed between MR element 404 and target 402 (notshown).

In the single coil configuration, MR element 404 may be subject to amagnetic field even in the absence of magnetic target 402. If magnetictarget 402 is absent, there will be no eddy current and no reflectedmagnetic field. However, because MR element 404 is placed proximate to asingle coil 406, and not placed between two opposing coils, it may besubject to a directly coupled magnetic field 405 produced by the coil406.

The presence of target 402 may result in a reflected magnetic field andthis additional field can be detected by MR element 404 to indicate thepresence of target 402. For example, current through coil 406 mayproduce eddy currents (shown by currents 408 and 410) in target 402,which may produce reflected magnetic field 412. Reflected magnetic field412 may increase the strength of the magnetic field experienced by MRelement 404. Thus, when target 402 is present, MR element 404 may detecta stronger magnetic field than when target 402 is absent.

The proximity of target 402 may also increase or decrease the strengthof the reflected magnetic field detected by MR element 404. As target402 moves closer to coil 406 (or vice versa), the eddy currents (shownby currents 408′ and 410′) will increase in strength, which will producea reflected magnetic field 412′ with greater strength. Thus, MR element404 will detect stronger magnetic field as target 402 moves closer tocoil 406.

In the embodiment shown in FIG. 4, MR element 404 is positioned adjacentto loops of coil 406. This may result in greater sensitivity of MRelement 404 to detect reflected field 412. However, because the fieldproduced by coil 406 is not zero at the position of MR element 404, MRelement 404 may also detect not only the reflected field, but also themagnetic field directly produced by the coil 406, i.e. a “directlycoupled” magnetic field. Various techniques may be used to reduce MRelement 404's sensitivity to the directly coupled magnetic field.

Referring to FIG. 5, circuit 500 includes a coil 502 and four MRelements 1-4 placed above or below traces of coil 502. The MR elementsmay be connected in a bridge configuration 504. The bridge configurationmay provide a differential output consisting of signals 504 a and 504 b.

In embodiments, circuit 500 may be used as a single-coil circuit fordetecting a target. For example, as a target approaches MR elements 1and 2, output signal 504 a may change, and as the target approaches MRelements 3 and 4, output signal 504 b may change. MR elements 1-4 may bealigned so that, as the target approaches elements 1-4, output signal504 a increase in value and output signal 504 b decreases in value, orvice versa. For example, in such embodiments, the field created by thecoil near the elements 1 and 2 is opposite is sign compared to the fieldcreated by the coil near the elements 3 and 4. Hence the reflectedfields are in opposite direction enhancing the sensitivity of the bridgedifferential output to the reflected field while suppressing thevariation due to external common fields.

Referring to FIG. 5A, circuit 500′ includes a coil 506 arranged so that,if current flows through coil 506 in the direction shown by arrow 508,the current will flow through coil portion 510 in a clockwise directionand through a counter-loop coil portion 512 in a counterclockwisedirection. Thus, coil portions 510 and 512 may produce local magneticfields having opposite direction, as described above. MR elements 1-4may be arranged as shown to form a bridge that provides a differentialsignal as the target approaches. The counter-loop may reduce thedirectly-coupled magnetic field produced by the coil and detected by theMR elements. For example, a magnetic field produced by coil 506 may bedirectly detected by (e.g. directly coupled to) MR elements 1-4. Coilportions 510 and 512 may each create a local magnetic field in theopposite direction of the magnetic field produced by coil 506. Thus, thelocal magnetic fields may (at least partially) cancel out the directlycoupled field produced by coil 506 at least in the local area around MRelements 1-4. This may reduce or eliminate the directly-coupled field asdetected by MR elements 1-4 so that the magnetic field detected by MRelements 1-4 is the reflected field from the target.

In embodiments, the counter-loop is used to measure reflected field andthe direct field of the coil to provide sensitivity detection. Also, inthis configuration, MR elements 1-4 can be placed so they do not see thefield created by the main coil.

In embodiments, the target may be positioned adjacent to MR elements 1and 3, but not 2 and 4 (or vice versa). If MR elements 1-4 are arrangedin a bridge formation, a differential output of the bridge may change asthe target moves toward or away from MR elements 1 and 3, for example.

In embodiments, the target may be positioned so that MR elements 1 and 2experience the reflected magnetic field in one direction (e.g.experience one side of the reflected magnetic field) and MR elements 3and 4 experience the reflected magnetic field in the opposite direction(e.g. experience the other side of the reflected magnetic field). Inthis embodiment, as the target moves closer to the MR elements, signal504 a may increase and signal 504 b may decrease (or vice versa) toproduce a differential signal.

Referring to FIG. 5B, circuit 500″ includes two MR bridges. MR bridge514 includes MR elements 1-4 and produces a differential output signalconsisting of signals 514 a and 514 b, whereas MR bridge 516 includes MRelements 508 and produces a differential output signal consisting ofsignals 516 a and 516 b. As a target approaches the MR elements 1-8, theoutput signals of MR bridges 514 and 516 may change to indicate thepresence and proximity of the target. Circuit 500″ is also shown withbond pads 518.

In an embodiment, the target may be positioned adjacent to bridge 514(MR elements 1-4) so that the differential output of bridge 514 isaffected as the target moves closer to or further from bridge 514. Inthis embodiment, the output of bridge 516 may remain relatively stableas the target moves. Thus, the output of bridge 516 may be used as areference. In particular, this arrangement may work in situations wherethe target to be detected is relatively close to bridge 514, so thatmovement of the target has a greater effect on bridge 514 and a smalleror zero effect on bridge 516.

Additionally or alternatively, the same configuration can be used tomeasure a difference of distance, the first distance being between alarge target and the lock of MR elements 1, 2, 3, and 4 and the seconddistance being between the corresponding target and MR elements 5, 6, 7,and 8.

Additionally or alternatively, the same configuration of FIG. 5B can beused to determine accurately the centering of a target along a planeperpendicular to the plane of the coil and crossing the plane of thecoil along the line 530 situated at equal distance between the bridges514 and 516.

Referring to FIG. 5C, circuit 501 includes a coil 520 and multiple MRelements 522 arranged at intervals around coil 520. MR elements 522 mayform a grid, similar to the grid described above and shown in FIG. 3B.In embodiments, MR elements 522 may be connected in bridgeconfigurations. In other embodiments, MR elements 522 may act (or bepart of) individual circuits that are not shared with other MR elements.In either case, MR elements 522 may produce a signal when a target (andits reflected magnetic field) are detected. A processor may receivethese signals and calculate the location, position, speed, parallelism,angle or other properties of the target.

In an embodiment, circuit 501 may be used to detect the position of thetarget in three dimensions with respect to the coil. Because the MRelements are positioned in a plane along coil 520, they may act as agrid. As the target approaches one (or more) of the MR elements, theywill produce an output signal that can be used to determine the locationof the target along the two dimensions of the grid. Also, as describedabove, coil 520 and the MR elements may be used to detect distance fromthe MR elements in a direction orthogonal to the two dimensions of thecoil and grid (i.e. a direction into and out of the page).

Referring now to FIG. 6, a circuit 600 for detecting a target mayinclude a coil 602 and one or more MR elements 604 and 606. Coil 602 mayhave two coiled portions 608 and 610, separated by a gap 612. Inembodiments, the current through portions 608 and 610 flows in the samedirection. For example, if the current through portion 608 flows in aclockwise direction around the coil, the current through portion 610 mayalso flow in a clockwise direction.

MR elements 604 and 606 may be placed within the gap so that they arenot directly above (or below) traces of coil 602. Placing MR elementswithin gap 612 may reduce capacitive or inductive coupling between coil602 and MR elements 604 and 606. Also, gap 612 may have a width W thatis smaller than the distance between the MR elements and the target. Asa result of gap 612 being relatively small, the eddy currents induced inthe target and the resulting reflected magnetic field may appear (i.e.may be detected by the MR elements) as if a single coil without any gapbetween portions were producing the magnetic field.

In embodiments, positioning MR elements within gap 612 may reducesensitivity of the MR elements to the directly coupled magnetic fieldproduced by gap 612, thus allowing the MR elements to maintainsensitivity to the reflected field.

In other embodiments, coil 602 may include a jog in one or more of thetraces. MR elements 604 and 606 may be aligned with the jog.

FIG. 7 is a cross-sectional view of a circuit having MR elements 604 and606 sandwiched between traces of coil 700. In an embodiment, coil 700may be the same as or similar to coil 602. Coil traces 602 a and 602 bmay be positioned on the surface of a substrate (not shown). MR elements604 and 606 may be placed atop traces 602 a and 602 b so that traces 602a and 602 b are positioned between MR elements 604 and 606 and thesubstrate. An additional layer of traces 614 a and 614 b may bepositioned atop MR elements 604 and 606. Traces 602 a, 602 b, 614 a, and614 b may be part of the same coil so that current flowing through thetraces flows in a circular or spiral pattern to induce a magnetic field.Placing MR elements 604 and 606 between traces of the coil may reducedirectly coupled magnetic field produced by the coil.

Referring to FIG. 8, a pressure sensor 800 includes a magnetic fieldsensor 802, having a substrate 803 that supports a coil 804 and MRelements 806 and 808. In embodiments, magnetic field sensor 802 may bethe same as or similar to circuit 500 in FIG. 5, circuit 300 in FIG. 3,or any of the magnetic field detection circuits described above that candetect proximity of a target.

In embodiments, coil 804 and MR elements 806, 808 may be supported bythe same substrate 803. In other embodiments, MR element 806, MR element808, and coil 804 may be supported on different substrates (not shown).For example, coil 804 may be supported by one substrate while MRelements 806 and 808 may be supported by a different substrate. Inanother example, MR element 806, MR element 808, and coil 804 may eachbe supported by a separate substrate. Any other combinations ofsubstrates supporting circuit elements are also possible.

Pressure sensor 800 includes a chamber 810 having a conductive portion811 and a deformable portion 812. In an embodiment, chamber 810 isformed by an elongate tube. In the embodiment of FIG. 8, the conductiveportion and the deformable portion 812 may comprise a membrane disposedat one end of the tube that can act a diaphragm, and can be deformed tomove toward or away from magnetic field detection circuit 802.

Deformable portion 812 may be formed of stainless steel, copperberyllium, titanium alloys, super alloys, and/or sapphire. When thepressure inside chamber 810 is greater than the pressure outside chamber810, deformable portion 812 may extend toward magnetic field detectioncircuit 802. If the pressure outside chamber 810 is greater, deformableportion 812 may retract away from magnetic field detection circuit 812,and if the pressure inside and outside chamber 810 is in equilibrium,deformable portion may adopt a neutral position between the extended andretracted positions.

In case of a circular deformable portion, the deformation of themembrane is given by the formula:

$d = {\frac{3}{16}\frac{p}{{Eh}^{3}}\left( {1 - v^{2}} \right)\left( {a^{2} - r^{2}} \right)^{2}}$

Where h is the thickness of the deformable portion, v is the Poissonmodule, E is the young module, a is the radius of the deformableportion, r is the point where the deformation is measured.

In embodiments, the maximal deformation may be low enough that thedeformable portion is always in the elastic domain of the material evenat temperature above 180° C. For that reason, super alloys like maragingalloys or titanium alloys may be suitable materials.

Magnetic field detection circuit 802 may include at least one magneticfield sensing element 806 and/or 808 disposed proximate to coil 804, asdescribed above. Coil 804 may produce a magnetic field that induces eddycurrent and a reflected magnetic field in the conductive portion 812,similar to the eddy currents and reflected fields described above.Magnetic field detection circuit 802 may also include a circuit togenerate an output signal indicative of the pressure differentialbetween the interior and exterior of chamber 810.

In embodiments, magnetic field detection circuit 802 comprises twospaced apart MR elements 806 and 808 and detects a distance between theconductive portion 812 and one of the MR elements 806 and 808 asdeformable portion extends toward and/or retracts away from the MRelements. In embodiments, magnetic field detection circuit 802 may beconfigured to detect a difference between a) the distance between theconductive portion 812 and magnetic field sensor 808, and b) thedistance between conductive portion 812 and magnetic field sensor 806.The difference between these distances may be used to produce an outputsignal of magnetic field detection circuit 802.

The output signal produced by magnetic field detection circuit 802 mayrepresent the distance, which can then be received by a processor tocalculate an associated pressure within chamber 810. MR elements 806 and808 may comprise multiple MR elements and may be arranged in a bridgeconfiguration, as described above, to produce a differential output.

In an embodiment, MR element 806 is aligned with an edge of conductive,deformable portion 812 and MR element 808 is aligned with the center ora central region of conducive, deformable portion 812. In thisarrangement, MR element 808 will react as deformable portion 812 movestoward and away from MR element 808, and MR element 806 will not beaffected or will be affected to a significantly lesser degree thanelement 808, and thus may have a relatively constant resistance value.Positioning the MR elements in this way may be used to remove errors dueto stray field. It may also help compensate for air gap tolerancebetween MR elements. For example, the difference in distance detected bythe the two sensors may be used to compensate for small changes in airgap over time, temperature, etc.

Referring to FIG. 8A, another embodiment of a pressure sensor 818includes a first elongated tube 820 having a deformable sidewall 821 andan opening 823 that allows a fluid to enter a chamber within elongatedtube 820. As the fluid creates pressure within tube 820, the sidewall821 may expand like a balloon or extend. An end 828 of tube 820 may beconductive.

Pressure sensor 818 also includes a second elongated tube 822 having anopening 824. Elongated tube 822 may have a rigid wall 826, and anopening 824. Opening 824 may have a diameter or size large enough fortube 820 to be inserted into opening 824.

Pressure sensor 818 may include a magnetic field sensor 830, which maybe the same as or similar to magnetic field sensor 802, and/or any ofthe magnetic field sensors described above.

In embodiments, when the tubes 820, 822 are assembled, conductive end828 of tube 820 may be positioned proximate to MR element 808. As thepressure within tube 820 increases and decreases, the rigid wall of tube822's may keep deformable sidewall 821 from expanding laterally.However, end 828 may expand and extend toward MR element 808 and retractaway from MR element 808 as pressure within tube chamber 823 changes.Magnetic field sensor 830 may detect the change in distance and producean output signal representing the distance between end 828 and MRelement 808. In embodiments, magnetic field detection circuit 802 may beconfigured to detect a difference between a) the distance betweenconductive end 828 and magnetic field sensor 808, and b) the distancebetween conductive 808 and magnetic field sensor 806. The differencebetween these distances may be used to produce an output signal ofmagnetic field detection circuit 830. A processor circuit may receivethe signal and calculate a pressure within tube 820 based on thedistance.

Referring also to FIG. 9, pressure sensor 900 includes a first substrate902, that may be the same or similar to substrate 803 of FIG. 8, and asecond substrate 904 attached to the first substrate 902. Secondsubstrate 904 may include a surface 908 and recess 906 formed in thesurface. Recess 906 may be etched into the substrate. In embodiments,wafer 904 may be etched so that it is thin enough to deflect underpressure, as shown by dotted lines 910. MR elements supported bysubstrate 902 may detect (via a reflected magnetic field as describeabove) the deflection of wafer 904. The detected deflection may besubsequently correlated to a pressure.

In embodiments, the MR elements on substrate 902 may be positioned sothat one or more MR elements are adjacent to an edge (e.g. anon-deflecting portion) of recess 906 and one or more MR elements areadjacent to the center (e.g. a deflecting portion) of recess 906,similar to the arrangement described above and illustrated in FIG. 8A.

In embodiments, substrate 904 may be formed from a conductive material,for example copper. Therefore, motion of a conductive deformable portionof substrate 904 caused by pressure on substrate 904 (and/or pressurewithin recess 906) can be detected by a magnetic field sensors onsubstrate 902.

Alternatively the substrate 904 may be formed by a crystalline materiallike sapphire coated by a thick enough conductive material like copperfor example.

In embodiments, recess 906 is evacuated during the manufacturing processto determine a reference pressure. In embodiments, the referencepressure is a vacuum or a pressure that is less than standard pressure(e.g. less than 100 kPa). In certain configurations, one or more of theoutput signals of an MR bridge (e.g. bridge 318 in FIG. 3A) may be usedto generate to represent the value of the reference pressure.

Referring to FIG. 10, a block diagram of a magnetic field sensor 1000 isshown. Magnetic field sensor includes a coil 1002 to produce a magneticfield, coil driver 1004 to provide power to the coil, MR element 1006,and MR driver circuit 1008 to provide power to MR element 1006. MRelement 1006 may be a single MR element or may comprise multiple MRelements, which may be arranged in a bridge configuration. As describedabove, coil 1002 and MR element 1006 may be configured to detect thedistance of a conductive target. In embodiments, coil driver 1004 and/orMR driver 1008 may produce an AC output to drive coil 1002 and MRelement 1008, as described above and as indicated by AC source 1010. ACsource 1010 may be a common source used to drive both coil 1002 and MRelement 1006. In embodiments, signal 1012 may be an AC signal.

Magnetic field sensor 1000 also includes an amplifier to amplify theoutput signal 1012 of MR element 1006. Output signal 1012 may be adifferential signal and amplifier 1014 may be a differential amplifier.Output signal 1012 and amplified signal 1016 may a DC signal.

Magnetic field sensor 1000 may also include a low pass filter 1018 tofilter noise and other artifacts from signal 1016, and an offset module1024 which may scale the output signal according to temperature (e.g. atemperature measure by temperature sensor 1020) and a type of materialaccording to material type module 1022. A segmented linearizationcircuit 1026 may also be included, which may perform a linear regressionon compensated signal 1028 and produce output signal 1030.

In embodiments, the reflected magnetic field from the target will have afrequency f (the same frequency as the coil driver 1004). Because themagnetic field produced by coil 1002 and the reflected field have thesame frequency, the output of MR element 1006 may include a 0 Hz (or DC)component, a component at frequency f, and harmonic components atmultiples of frequency f. One skilled in the art will recognize that thelowest frequency harmonic component may occur at frequency 2*f. However,any difference in the equilibrium of the MR bridge may generate afrequency component that may be present in the signal. Thus, low passfilter 1018 may be configured to remove the frequency f and higher (i.e.low pass filter 1018 may include a cut-off frequency fcutoff, wherefcutoff<f. In embodiments, the filter may be designed to remove possiblef signals. Accordingly, the frequency f may be chosen as a frequencygreater than the frequency range of motion of the target.

In embodiments, the sensitivity of MR element 1008 changes withtemperature. The strength of the reflected field may also change withtemperature depending of target material type and frequency. Tocompensate, module 1022 may contain parameters to compensate for theeffects of the temperature and/or material used. The parameters mayinclude linear and/or second order compensation values.

In embodiments, processing circuit 1032 may process the signalrepresenting the magnetic field. Because a common source 1010 is used todrive MR element 1006 and coil 1002, the frequency of coil 1002 and MRelement 1006 is substantially the same. In this case, post processing ofthe signal may include filtering, linear regression, gain andamplification, or other signal shaping techniques.

MR element 1006 may detect the magnetic field directly produced by coil1002 and also the reflected magnetic field produced by eddy currents ina conductive target, induced by the magnetic field generated by currentthrough coil 1002.

Referring to FIG. 10A, magnetic field sensor 1000′ may include coil1002, coil driver 1004, common AC source 1010, MR driver 1008, MRelement 1006, amplifier 1014, and low pass filter 1018 as describedabove.

Magnetic field sensor 1000′ may differ from sensor 1000 of FIG. 10 inthat it is a closed loop sensor and so may also include a second coil1035, which may operate at a different AC frequency than coil 1002. Inthis example, coil 1035 may be 180 degrees out of phase with coil 1002as indicated by the “-f” symbol. Coil 1035 may also produce a firstmagnetic field that can be used to detect a target. In embodiments, coil1035 may be relatively smaller than coil 1002. Coil 1035 may be placedadjacent to MR element 1006 to produce a magnetic field that can bedetected by MR element 1006, but which does not produce eddy currents inthe target.

In embodiments, coil 1035 may be used to offset errors due to themagnetoresistance of the MR element. For example, the magnitude ofcurrent driven through coil 1035 may be changed until the output of MRelement 1006 is zero volts. At this point, the current through coil 1035may be measured (for example, by measuring voltage across a shuntresistor in series with coil 1035). The measured current may beprocessed similarly to the output of MR element 1006 to remove amagnetoresistance error associated with MR 1006.

Magnetic field sensor 1000′ may also include an amplifier 1036 toreceive signal 1038. Magnetic field sensor 1000′ may also include lowpass filter 1019, material type module 1022, temperature sensor 1020,offset module 1024, and segmented linearization module 1026 as describedabove.

FIGS. 11-11F include various examples of magnetic field sensors havingsignal processing to reduce inductive coupling or other noise fromaffecting signal accuracy. The example magnetic field sensors in FIGS.11-11F may also employ various features related to detecting a reflectedfield from a target, such as frequency hopping, etc. Such magnetic fieldsensors may also include circuitry to compute a sensitivity value.

Referring now to FIG. 11, a magnetic field sensor 1100 may include coil1002, coil driver 1004, AC driver 1010, MR driver 1008, MR element 1006,amplifier 1014, low pass filter 1018, temperature sensor 1020, materialtype module 1022, offset module 1024, and segmented linearization module1026.

MR element 1006 may be responsive to a sensing element drive signal andconfigured to detect a directly-coupled magnetic field generated by coil1002, to produce signal 1012 in response. Processing circuitry maycompute a sensitivity value associated with detection, by MR element1006, of the directly-coupled magnetic field produced by coil 1002. Thesensitivity value may be substantially independent of a reflected fieldproduced by eddy currents in the target.

As shown, AC driver 1010 is coupled to coil driver 1004, but is notcoupled to MR driver 1008 in sensor 1100. In this embodiment, MR driver1008 may produce a DC signal (e.g. a signal with a frequency of aboutzero) to drive MR element 1006.

Coil 1002 may produce a DC (or substantially low frequency AC) magneticfield that can be detected by MR element 1006, but which does notproduce eddy currents in the target. The signal produced by detection ofthe DC (or substantially low frequency AC) magnetic field may be used toadjust sensitivity of the magnetic field sensor.

Coil 1002 may also produce an AC magnetic field at higher frequenciesthat induces eddy currents in the target, which produce a reflectedmagnetic field at those higher frequencies that can be detected by MRelement 1006.

MR element 1006 may produce signal 1012, which may include frequencycomponents at the DC or substantially low AC frequency (e.g. a “directlycoupled” signal or signal component) representing the lower frequencymagnetic field that does not cause eddy currents in the target, and/orfrequency components at the higher AC frequency (e.g. a “reflected”signal or signal component) that represent the detected reflected field.The directly coupled signals may be used to adjust sensitivity of thesensor while the reflected signals may be used to detect the target.Coil driver 1004 and/or MR driver 1008 may use the directly coupledsignals as a sensitivity signal adjust their respective output drivesignals in response to the sensitivity signal.

In embodiments, the directly coupled signal and the reflected signal maybe included as frequency components of the same signal. In this case,coil 1002 may be driven to produce both frequency components at the sametime. In other embodiments, generation of the directly coupled signaland the reflected signals may be generated at different times, forexample using a time-division multiplexing scheme.

Sensor 1100 may also include a demodulator circuit 1050 that canmodulate signal 1016 to remove the AC component from the signal or shiftthe AC component within the signal to a different frequency. Forexample, demodulator circuit 1050 may modulate signal 1016 at frequencyf. As known in the art, because signal 1016 includes signal componentsat frequency f representing the detected magnetic field, modulatingsignal 1016 at frequency f may shift the signal elements representingthe detected magnetic field to 0 Hz or DC. Other frequency componentswithin signal 1016 may be shifted to higher frequencies so they can beremoved by low-pass filter 1018. In embodiments, the DC or low frequencycomponent of signal 1016, which may represent a sensitivity value, canbe fed back to coil driver 1004 to adjust the output of coil 1002 inresponse to the signal, and/or to MR driver 1008 to adjust drive signal1009 in response to the sensitivity value. DC output signal 1052 mayrepresent proximity of the target to MR element 1006.

In other embodiments, a time-division multiplexing scheme may be used.For example, coil driver 1004 may drive coil 1002 at a first frequencyduring a first time period, at a second frequency during a second timeperiod, etc. In some instances, the first and second (and subsequent)time periods do not overlap. In other instances, the first and secondtime periods may overlap. In these instances, coil driver 1004 may drivecoil 1002 at two or more frequencies simultaneously. When the first andsecond time periods do not overlap, demodulator 1050 may operate at thesame frequency as the coil driver 1004. When the time periods overlap,multiple modulators can be used, the first running at the firstfrequency, and the second running at the second frequency in order toseparate out the signals at each frequency.

While it can be advantageous to reduce the directly coupled magneticfield that the MR element 1006 detects in order to achieve an accurateread of the reflected field (and thus the detected target), it may alsobe advantageous to have some amount of direct coupling (i.e., todirectly detect the magnetic field produced by coil 1002) to permit asensitivity value to be computed. The simultaneous measure of both thefield reflected and the field created by the coil allows to determineaccurately the distance of the object independent of the sensitivity ofthe MR elements, coil drive current, etc. . . . The sensitivity of MRelements may vary with temperature and/or with the presence of unwantedDC or AC stray fields in the plane of the MR array. The ratio betweenthe reflected field and the coil field is just dependent on geometricaldesign and is hence a good parameter to accurately determine a distance.

Referring to FIG. 11, a frequency hopping scheme may be used. Forexample, coil driver 1004 may drive coil 1002 at different frequencies(e.g. alternate between frequencies over time, or produce a signalcontaining multiple frequencies). In such embodiments, sensor 1100 mayinclude multiple demodulator circuits and/or filters to detect a signalat each frequency.

Referring to FIG. 11A, a magnetic field sensor 1100′ includes coil 1002,coil driver 1004, AC driver 1010, MR driver 1008, MR element 1006,amplifier 1014, low pass filter 1018, temperature sensor 1020, materialtype module 1022 and offset module 1024.

As shown, AC driver 1010 is coupled to coil driver 1004 to drive coil1002 at a frequency f1. MR driver 1008 is coupled to AC driver 1102 todrive MR element 1006 at a frequency f2. Frequencies f1 and f2 may bedifferent frequencies and may be non-harmonic frequencies (in otherwords, f1 may not be a harmonic frequency of f2 and vice versa). Inembodiments, frequency f1 is lower than frequency f2. In otherembodiments, frequency f1 and f2 may be relatively close to each otherso that the difference between the two frequencies falls well below f1and f2. Frequency f2 may be a zero value or non-zero-value frequency butalternatively, we may choose f1 larger than f2. Then the demodulation isdone at f2−f1.

In an embodiment, frequency f1 may be selected to avoid generating aneddy current greater than a predetermined level in the target and/orselected to provide full reflection in the target. The reflected fieldmay be related to the skin depth in the target according to thefollowing formula:

$\delta = \frac{1}{\sqrt{{\sigma\mu\pi}\; f}}$

In the formula above, a is the conductivity of the target material, μ isthe magnetic permittivity of the target material, and f is the workingfrequency. If the thickness of the target material is larger than about5 times the skin depth δ, the field may be totally reflected. In thecase where the thickness of the target is equal to the skin depth, onlyabout the half of the field may be reflected. Hence a frequency f chosento be low enough so the skin depth becomes larger than the thickness ofthe target may induce low eddy currents and a reflected field withreduced strength. The formula given above may be valid for highconductive and low magnetic materials. For material with lowconductivity or for ferromagnetic material, losses of the eddy currents,which may be translated at a complex skin depth, may result in reductionof reflected field strength.

Circuit 1100′ may also include a band pass filter 1104 and a demodulatorcircuit 1106. Band pass filter 1104 may have a pass band that excludesfrequencies f1 and f2 but conserves frequency |f1−f2|. In this way,inductive noise from the coil and/or GMR driver into the magneticsensors may be filtered out. Circuit 1100′ may also include ademodulator circuit 1106 that demodulates at frequency |f1−f2| and a lowpass filter to recover a signal centered around DC, which may representthe magnetic field seen by the magnetic sensors at f1. In embodiments,the signal at frequency |f1−f2| may include information about the targetand/or the directly coupled magnetic field, but may have reduced noisefrom inductive coupling or other noise sources.

Referring now to FIG. 11B, a magnetic field sensor 1100″ includes coil1002, coil driver 1004, AC driver 1010, MR driver 1008, MR element 1006,amplifier 1014, low pass filter 1018, temperature sensor 1020, materialtype module 1022, offset module 1024, and segmented linearization module1026.

As shown, AC driver 1010 is coupled to coil driver 1004, but is notcoupled to MR driver 1008 in sensor 1100. In this embodiment, MR driver1008 may produce a DC signal (e.g. a signal with a frequency of aboutzero) to drive MR element 1006.

Coil 1002 may produce an AC magnetic field that induces eddy currentsand a reflected magnetic field in a target.

Sensor 1100″ may also include a demodulation circuit 1060 that candemodulate signal 1016. Demodulation circuit 1060 may multiply signal1016 by a signal at frequency f, which may shift information about thetarget in signal 1016 to DC, and may shift noise or other information inthe signal to higher frequencies. Low pass filter 1018 may the removethe noise at higher frequencies from the signal. In embodiments,demodulation circuit 1060 may be a digital circuit that demodulatessignal 1016 in the digital domain or an analog signal the demodulatessignal 1016 in the analog domain.

Sensor 1100″ may also include a phase detection and compensation circuit1062 that detects the phase and/or frequency of the current in coil 1002and the magnetic field it produces. Circuit 1062 may detect andcompensate for discrepancies in phase in coil 1002 and f and produce acorrected signal 1063 that can be used to modulate signal 1016.

In embodiments, the frequency f, the type of material of the target, theshape of the target, wiring and electronics, and/or other factors maycause a phase shift between the drive signal 1010 to coil 1002 and thereflected magnetic field detected by MR element 1008. The phase betweenthe signals can be measured and used to adjust the phase of signal 1063from phase detection and compensation circuit 1062 to match the phase ofsignal 1016.

A frequency hopping scheme may also be used. For example, coil driver1004 and/or MR driver 1008 may drive signals at multiple frequencies. Ateach frequency, phase detection and compensation module 1062 may adjustthe phase of signal 1063 to match the phase of signal 1016.

Referring now to FIG. 11C, a magnetic field sensor 1100′″ includes coil1002, coil driver 1004, AC driver 1010, MR driver 1008, MR element 1006,amplifier 1014, temperature sensor 1020, material type module 1022,offset module 1024, and segmented linearization module 1026.

As shown, AC driver 1010 is coupled to coil driver 1004, but is notcoupled to MR driver 1008 in sensor 1100. In this embodiment, MR driver1008 may produce a DC signal (e.g. a signal with a frequency of aboutzero) to drive MR element 1006.

Coil 1002 may produce an AC magnetic field that induces eddy currentsand a reflected magnetic field in a target. The reflected magnetic fieldcan be detected by MR element 1006, which produces signal 1012representing the detected magnetic field.

Sensor 1100′″ may also include a fast Fourier transform (FFT) circuit1070 that can perform an FFT on signal 1016. Performing the FFT mayidentify one or more frequency components in signal 1016. In anembodiment, FFT circuit 1070 may identify the frequency component withthe greatest amplitude in signal 1016, which may represent the detectedmagnetic field at frequency f. FFT circuit 1070 may produce an outputsignal 1072 including the detected signal at frequency f, as well as anyother frequency components of signal 1016.

Alternatively, the driver can generate simultaneously differentfrequencies fa, fb, fc, and the FFT module may calculate the amplitudesat fa, fb, fc, which may be used to determine different parameters ofthe target including position, material, thickness, etc. In addition, ifa disturbance (e.g. from a deformation of the target, a stray magneticfield, a noise source, etc.) occurs at a particular frequency, thesystem can detect the disturbance and ignore data at that frequency. Theamplitudes calculated by the FFT module may also be used to determine ifthere is a disturbance at any particular frequency, which can be ignoredby subsequent processing. In embodiments, he FFT temperature gaincompensation and linearization may be calculated in the analog and/ordigital domain.

Referring now to FIG. 11D, a magnetic field sensor 1100D includes coil1002, coil driver 1004, MR driver 1008, and MR element 1006. The outputsignal 1007 of MR sensor 1006 may represent a detected magnetic field.Although not shown, sensor 1100D may also include amplifier 1014, lowpass filter 1018, temperature sensor 1020, material type module 1022,offset module 1024, and segmented linearization module 1026. Anoscillator 1182 may be used to operate coil driver 1004 at a frequencyf.

As shown, oscillator 1182 is coupled to coil driver 1004, but is notcoupled to MR driver 1008 in sensor 1100D. In this embodiment, MR driver1008 may produce a DC signal (e.g. a signal with a frequency of aboutzero) to drive MR element 1006.

Sensor 1100D also includes a quadrature demodulation circuit 1180.Quadrature demodulation circuit 1180 includes shift circuit 1188 toproduce a 90° shift of the driving frequency f. Oscillator 1182 mayproduce a cosine signal at frequency f. Thus, the output of 1188 may bea sine signal at frequency f. Hence by a multiplication in thedemodulators 1190 and 1192 (and subsequent low pass filtering), thedetected signal of the MR sensor 1006 may be separated into in-phase andout-of-phase components (e.g. signals 1184 a and 1186 a). The resultingphase and magnitude can be used to determine information about thereflected field and the target. For example, phase information may beused to determine if there is a defect or abnormality in the target, todetermine magnetic properties of the material of the target, whether thetarget is aligned properly, etc. Oscillator 1182 may also produce asquare wave with period 1/f, and shift circuit 1188 may shift the squarewave in time by 1/(4f).

Referring to FIG. 11E, in another embodiment, magnetic field sensor1100E may produce a quadrature modulated signal via two signal paths asan alternative to providing both the in-phase and out-of-phaseinformation. In circuit 1100E, half of the MR elements may be driven bya signal at frequency f and half of the MR elements may be driven with afrequency 90° out of phase. The demodulation chain (e.g. the circuitsthat comprise a demodulation function of the system) may be the same asor similar to the demodulation circuits in FIG. 10 including a low passfilter at DC and compensation and linearization.

In embodiments, quadrature modulation may be used to determine theabsolute magnitude and phase of the returned signal. This may allow forautomatic correction of unwanted dephasing of the signal, which mayprovide a more accurate determination of target properties and retrievalof information related to magnetic or loss properties of the material.

Referring to FIG. 11F, a magnetic field sensor 1100F includes coildriver 1004 that drives coil 1002 at a frequency of f₁. MR driver 1008may drive MR element at the same frequency f₁, but 90 degrees out ofphase with respect to coil drive 1004. As a result, the signal 1016produced by MR element 1006 may have a frequency that is two times f₁(i.e. 2*f₁), which may be a result of multiplying a sine and a cosine.Sensor 1100F may include a demodulator circuit 1195 that may demodulatethe signal to convert the reflected field information to a frequencyaround DC.

Referring to FIG. 12, signal 1270 may represent a signal used by coildriver 1004 to drive coil 1002. When the signal is high, coil driver1004 may drive coil 1002 with current flowing in one direction, and whenthe signal is low, coil driver may drive coil 1002 with current flowingin the opposite direction. In embodiments, coil driver 1004 may drivecoil 1002 with direct current (i.e. at DC) or at a frequencysufficiently low so that the magnetic field produced by coil 1002 doesnot create eddy currents in the target.

As an example, referring to the skin depth formula above, the skin depthof copper at r 50 Hz may be about 10 mm and at 10 kHz it may be about600 μm. Hence, given a 0.5 mm thick copper target, a frequency below 5kHz may create reflected magnetic fields with relatively low strength.

Coil driver 1004 may drive coil 1002 at a relatively low or DCfrequency, as shown by signal portions 1272 and 1274. The frequency maybe sufficiently low, and thus the duration of portions 1272 and 1274 maybe sufficiently long, so that any eddy currents generated in the targetby switching of signal 1270 (for example, switching from a high valueduring portion 1272 to a low value during portion 1274) have time tosettle and dissipate. The directly-coupled signal shown during portions1272 and 1274 may switch from high to low (representing a change in thedetected magnetic field) in order to remove any offset due to thedirectly-coupled magnetic field of coil 1002.

Portion 1276 of signal 1270 may represent the magnetic field detected byMR element 1006 while coil driver 1004 drives coil 1002 at a frequencysufficiently high to induce eddy currents in the target. While portion1276 is active, MR element 1006 may detect the directly-coupled magneticfield produced directly by coil 1002, and also the magnetic fieldproduced by eddy currents in the target. The detected signal may besubsequently processed to separate the directly-coupled field from thefield produced by the eddy currents. Although not shown, portion 1276may have a larger or smaller magnitude than portion 1272 because theportions may contain different information. For example, portion 1276may include the reflected signal as well as the directly-coupled signal.

As shown in signal 1270, low frequency portions 1272 and 1274 ofdifferent polarities may be adjacent to each other within signal 1270.In other embodiments, as shown in signal 1270′, low frequency portions1272′ and 1274′ of different polarities may not be adjacent to eachother within the signal. For example, they may be separated by highfrequency signal portion 1276.

In other embodiments, the coil may be driven at both the low frequency(of low frequency portions 1272 and 1274) and at the high frequency (ofhigh frequency portion 1276) simultaneously. The frequencies may then beseparated using signal processing techniques to measure a MR element'sresponse.

In certain instances, the ration of the low frequency portions 1272 and1274 to the high frequency portion 1276 can be used to determine orindicate the magnitude of the reflected signal. Measuring the ratio inthis way may reduce sensitivity of the magnitude measurement toexternal, unwanted variations such as variations due to temperate, straymagnetic fields, etc.

Referring now to FIG. 12A, magnetic field sensor 1200 may be configuredto adjust the output signal of the magnetic field sensor in response tothe sensitivity value. Sensor 1200 may include a coil 1202 and coildriver 1204. MR element 1206 may detect a magnetic field produced bycoil 1202, and as reflected by a target, as described above. Inembodiments, the output signal 1208 of MR element 1206 may comprise afirst frequency and a second frequency. For example, the first frequencymay be the frequency of the coil driver, and the second frequency may be0 Hz, or DC. In this case, MR element 1206 may be driven by a DC biascircuit 1210. In other examples, the second frequency may be a non-zerofrequency.

In another embodiment, coil driver 1204 may drive coil 1202 at onefrequency during a first time period, and by another frequency during asecond time period. The time periods may alternate and not overlap.

Sensor 1200 may also include a separator circuit, which may include oneor more low pass filters 1214 and 1216, as well as demodulators 1224 and1226. Sensor 1200 may also include mixer circuit 1212. Oscillators 1218and 1220 may provide oscillating signals used to drive coil 1202 andprocess signal 1208. In embodiments, oscillator 1220 may provide asignal with a higher frequency (f_(high)) than that of oscillator 1218(f_(low)). In embodiments, f_(low) is a sufficiently low frequency sothat any reflected field produced by the target as a result of frequencyflow is zero, sufficiently small that it is not detected, orsufficiently small so that its effect on the output is negligible orwithin system tolerances.

Mixer 1212 may mix (e.g. add) the signals from oscillator 1218 and 1220to produce signal 1222, which it feeds to coil driver 1204. Coil driver1204 may then drive coil 1202 according to the mixed signal 1202.

Because coil 1202 is driven by the mixed signal, output signal 1208 mayinclude oscillations at f_(high) and f_(low) as detected by MR sensor1206. Demodulator 1226 may demodulate signal 1208 at frequency f_(high)in order to separate the portion of signal 1208 at frequency f_(high)from other frequencies in the signal. One skilled in the art mayrecognized that the demodulation process may result in the otherfrequencies being shifted to higher frequencies in the signal. Low passfilter 1214 may then remove these frequencies from the signal andproduce a filtered signal 1228 comprising primarily information atfrequency f_(high) or at DC.

Similarly, demodulator 1224 may demodulate signal 1208 at frequencyf_(low) in order to separate the portion of signal 1208 at frequencyf_(low) from other frequencies in the signal. One skilled in the art mayrecognized that the modulation process may result in the otherfrequencies being shifted to higher frequencies in the signal. Low passfilter 1216 may then remove these frequencies from the signal andproduce a filtered signal 1230 comprising primarily information atfrequency f_(low) or at DC. Processing circuit 1232 may process signals1228 and 1230 to produce output signal 1232 representing the detectedtarget.

Processing circuit 1232 may process signals 1228 and 1230 in variousways including, taking the ratio of the signals to provide an outputthat is substantially insensitive to undesirable variations caused bystray magnetic field interference, temperature shifts, package stresses,or other external stimuli. Taking the ratio of the signals can alsoprovide an outout that is substantially insensitive to variations in thecoil driver (e.g. variations in current or voltage provided by the coildriver) due to temperature, changes in supply voltage, external stimuli,etc.

Signal 1230 may also be used as a sensitivity signal fed into DC biascircuit 1220, as shown by arrow 1234. DC Bias circuit 1210 may adjustthe voltage level used to drive MR element 1206 based on the value ofsignal 1230, to compensate for changes in system sensitivity due totemperature, stray magnetic fields, package stress, etc.

Referring to FIG. 12B, magnetic field sensor 1200′ may be similar tosensor 1200, and may also include an additional in-plane field coil1236. DC bias circuit 1236 may drive coil 1232 with a DC current tocreate a constant magnetic field. The constant magnetic field may bedetected directly by MR element 1206 and may be a biasing magneticfield. In other embodiments, the magnetic field produced by in-planefield coil 1232 may be used to generate a signal proportional to the MRsensitivity, which can be detected by MR element 1206 and subsequentlyfed back and used to adjust the sensitivity of circuit 1200′. Inembodiments, the magnetic field produced by in-plane field coil 1232 maybe perpendicular to the magnetic field produced by coil 1202 and used toincrease/decrease the sensitivity of the MR element. DC bias circuit1236 may drive coil 1232 in such a way to compensate for changes insensitivity seen by the closed loop system. In other words, DC biascircuit may change the magnitude of the driving current supplied to coil1232 in response to feedback signal 1234 to compensate for sensitivityerrors up to the bandwidth of the feedback loop system. The bandwidthmay be determined (or at least heavily influenced) by the cutofffrequency of filter 1216.

As shown, DC bias circuit 1236 may receive signal 1230 and adjust theamount of current provided to in-plane field coil 1232, which maysubsequently adjust thus the strength the magnetic field produced byin-plane field coil 1232. Although not shown in FIG. 12B, DC biascircuit 1210′ may also receive signal 1230 and use it to adjust thecurrent that drives MR element 1206. In embodiments, DC bias circuit1210′, DC bias circuit 1236, or both may adjust their outputs based onsignal 1230.

Referring to FIG. 12C, a magnetic field sensor 1240 includes oscillator1220, oscillator 1218, and mixer 1212. Coil driver 1204 receives thesignal produced by mixer 1212 and drives coil 1202 with a signalcomprising frequencies f_(high) and f_(low).

Sensor 1240 may include two (or more) MR elements 1254 and 1256. MRdriver 1250 may be coupled to oscillator 1220 and may drive MR sensor1254 at frequency f_(high), and MR driver 1252 may be coupled tooscillator 1218 and my driver MR sensor 1256 at frequency f_(low). Lowpass filter 1216 may filter output signal 1258 from MR sensor 1254 andlow pass filter 1264 may filter output signal 1260 from MR sensor 1256.Due to the frequencies at which MR sensors 1254 and 1256 are driven,output signal 1258 may include a frequency component at f_(high) andoutput signal 1260 may include a frequency component at f_(low).Filtered signal 1230 may be a sensitivity signal that can be used toadjust the sensitivity of sensor 1240. Thus, signal 1230 may be fed backto MR driver 1252, MR driver 1250, and/or coil driver 1204, which mayeach adjust their output based on the value of signal 1230. Inembodiments, signal 1230 may be a DC or oscillating signal.

Referring to FIG. 13, a circuit 1300 includes a coil 1302 and MRelements 1-8 arranged in bridge configurations. Coil 1302 may include socalled countercoil portions 1304A, B and 1306A, B. First countercoilportion 1304A may produce a field to the left for MR elements below it.Subsequently, portion 1304B may produce a field to the right, portion1306A may produce a field to the right, and portion 1306B may produce afield to the left. MR elements 1 and 3 are positioned near countercoilportion 1304A and MR elements 2 and 4 are positioned near countercoilportion 1304B. MR elements 5, 6 are positioned near countercoil portion1306A, and MR elements 7, 8 are positioned near countercoil portion1306B. Also, the MR bridges are split so that some of the elements ineach bridge are located near countercoil portion 1304 and some of theelements are located near countercoil portion 1306. For example, MRbridge 1308 comprises MR elements 1 and 3 (positioned near countercoilportion 1304) and MR elements 5 and 6 (positioned near countercoilportion 1306). Providing countercoil portions 1304 and 1306 mayinfluence the magnitude and polarity of the directly coupled field onthe MR elements.

MR elements 1, 3 may have a first coupling factor with relation to coil1302, MR elements 2, 4 may have a second coupling factor, MR elements 5and 6 may have a third coupling factor, and MR elements 7, 8 may have afourth coupling factor with relation to coil 1302. In an embodiment, thecoupling factor of MR elements 1, 3, 7, and 8 may be equal and oppositeto the coupling factor of MR elements 2, 4, 5, and 6. This may be due,for example, to coil portions 1304A, B and 1306A, B carrying equalcurrent in opposite coil directions, as well as the positioning of theMR elements in relation to them.

In an embodiment, bridges 1308 and 1310 will respond to a reflectedfield equally. However, they may respond oppositely to the directlycoupled field. The addition of the outputs of the two bridges maycontain information about the reflected field and the subtraction of thetwo bridges may contain information about the directly coupled field.The directly coupled field information can then be used as a measure ofsystem sensitivity and be used to normalize the reflected fieldinformation. In another embodiment, bridges 1308 and 1310 respond to areflected field equally. However, they may respond differently (notnecessarily exactly oppositely) to the directly coupled field. Thesubtraction of the two bridges still results in a signal only containinginformation about the directly coupled field, which can be used as ameasure of system sensitivity. The addition of the two bridges mayinclude some directly coupled field information along with informationabout the reflected field. However, this can be compensated for with thelinearization block, as it shows up as a constant offset.

For example, during operation, the following formulas may apply:

V _(bridge1)=(C _(r) +C ₁)*i*S ₁

V _(bridge2)=(C _(r) +C ₂)*i*S ₂

In the formulas above, C_(r) represents the reflected field, C₁represents the directly coupled field detected by the first MR bridge,C₂ represents the directly coupled field detected by the second MRbridge, i is the current through the coil, S₁ represents the sensitivityof the first MR bridge, and S₂ represents the sensitivity of the secondMR bridge. Assuming that S1=S2 and solving for Cr:

$C_{r} = \frac{{\left( V_{{bridge}\; 2} \right)\left( C_{1} \right)} - {\left( V_{{bridge}\; 1} \right)\left( C_{2} \right)}}{V_{{bridge}\; 1} - V_{{bridge}\; 2}}$

The equation above provides a formula for C_(r) independent of currentand sensitivity of the MR elements. In embodiments, the geometry of thecoil, MR elements, and target my provide that C₁=−C₂. In otherembodiments, the geometry of the system may provide other ratios of C₁and C₂. With a known ratio, C_(r) can be computed to provide a value forthe reflected field.

Referring to FIG. 13A, a coil 1302′ may include countercoil portions1304′A, B and 1306′A, B and gap between coil elements. In FIG. 13A, onlythe middle portion of coil 1302′ and MR elements 1-8 are shown.

The countercoil portions 1304′ and 1306′ may each be placed in arespective gap 1350 and 1352 between traces of the main coil. MRelements 1-8 may be placed within the gaps of the main coil. As with thegap in FIG. 6, placing the MR elements within gaps 1350 and 1350 mayreduce sensitivity of the MR elements to the directly coupled magneticfield. Thus, a coil design for coil 1302′ may adjust sensitivity of theMR elements to the directly coupled field by including gaps 1350 and1352 to reduce the sensitivity and countercoil portions 1304′ and 1306′to increase the sensitivity in order to achieve the desired directcoupling on each element. In an embodiment, the direct coupling field issimilar in magnitude to the reflected field.

Referring to FIG. 13B, magnetic field sensor 1320 may include coil 1302,MR bridge 1308, and MR bridge 1310 as arranged in FIG. 13. Coil driver1322 may drive coil 1302 at frequency f. MR driver 1324 may drive one orboth MR bridges 1308 and 1310 at 0 Hz (i.e. DC) or at another frequency.

Demodulator 1324 and demodulator 1326 may demodulate the output signalsfrom MR bridges 1308 and 1310, respectively, at frequency f. This mayshift the frequency components of the signals at frequency f to 0 Hz orDC, and may shift other frequency components in the signal to higherfrequency bands. Low pass filters 1328 and 1330 may the remove thehigher frequency components from the signals and provide a DC signal V1(corresponding to the magnetic field detected by MR bridge 1308 and a DCsignal V2 (corresponding to the magnetic field detected by MR bridge1310) to processing block 1332. Processing block 1332 may processsignals V1 and V2 to produce a signal representing the detected target.In an embodiment, processing block may perform the operationX=(V1+V2)/(V1−V2), where X is the signal representing the detectedtarget. In this embodiment, the position of the MR of the bridges 1308and 1310 are chosen in a way that the first bridge sees a negativesignal from the coil (directly coupled field) and the second an oppositesignal from the coil. Both bridges may see the same reflected signal.Hence V1+V2 may substantially comprise the reflective signal and V1−V2the coil signal. The ratio gives then a quantity X which is independenton the sensitivity change of the MR elements due to the temperature orstray fields for example, as well as variations in coil current. In thisembodiment, the position of the MRs (and/or coils) may be chosen so thateach MR is seeing (e.g. can detect) a coil signal and a reflected signalof the same range of amplitude i.e. typically a reflected field varyingfrom 0.1% to 100% of the direct detected field.

Referring now to FIG. 14, system 1400 includes a magnetic field sensor1402 and target 1404. Magnetic field sensor 1402 may be the same as orsimilar to magnetic field sensor 100 and/or any of the magnetic fieldsensors described above. Accordingly, magnetic field sensor 1402 mayinclude a coil to produce a magnetic field and produce eddy currentswithin conductive target 1404, and one or more magnetic field sensingelements to detect a reflected field from the eddy currents.

The skin effect of target 1404 may be used to detect linear, speed, andangle (in the case of a rotating target) measurements by controlling theamount of reflected magnetic signal, and using the amount of reflectedsignal to encode the target position. A target can be created bycombining a high conductivity material (shallow skin depth, measuredwith high frequency signal) and relatively low-conductivity materials(deep skin depth, measured using a medium or low frequency signal). Thetarget can be created by milling or etching a linear slope or digitaltooth pattern into the low conductivity material. In a subsequent step ahigh conductivity material can be deposited over the surface then milledor polished to create a planar surface. Alternatively, the lowconductivity material can be omitted.

Measurement techniques can also utilize various frequencies (of coil1002 for example) and the skin effect of the target. A relatively highfrequency and shallow skin depth can be used to measure the air gapdistance between the sensor and the face of the target. This signal canthen be used to calibrate the sensitivity of the system. A mediumfrequency with a skin depth that exceeds the maximum thickness of thehigh conductivity material may be used to sense the position of theportion of the target formed by the low conductivity material. Arelatively low frequency signal (e.g. low enough that it is notreflected by the target) may be used to measure the overall sensitivityof the MR sensors and provide feedback to compensate for any changes insensitivity due to stray field, temperature, or package stresses.Referring again to FIG. 14, target 1404 may comprise a first materialportion 1406 and a second material portion 1408. First material portion1406 may be a high-conductivity material, such as a metal; and secondmaterial portion 1408 may be a relatively low-conductivity material,such as a plastic, ceramic, or other insulating material; or vice versa.In embodiments, the first and second material portions 1406 and 1408 maybe a unitary structure as may be integrally formed or may be separateelements physically coupled to each other, as shown in FIG. 14.

The thickness 1410 of first material portion 1406 may vary along thelength of target 1404 so that, at one end 1412, first material portion1406 is relatively thick and, at another end 1414, first materialportion 1406 is relatively thin. The eddy currents induced by magneticfield sensor 1402 at the thick end 1412 of first material portion 1406may differ from those induced at the thin end 1414. Accordingly, thereflected magnetic field produced at thick end 1406 may also differ fromthe reflected magnetic field produced at thin end 1414. Because thethickness of first material portion 1406 varies linearly along thelength of target 1404, the reflected magnetic field may also varylinearly along the length of target 1404. Thus, the magnetic fieldsensing elements of magnetic field sensor 1402 may detect the differencein the reflected magnetic field to determine where magnetic field sensor1402 is positioned along the length of target 1404. In embodiments, if arelatively high frequency is used to sense the airgap, the thickness atend 1414 may be chosen to be greater than one skin depth and less thanfive skin depths at the chosen frequency. The thickness at end 1412 maybe chosen to be than one skin depth at a relatively lower frequency.

In embodiments, target 1404 may move in a linear direction (shown byarrow 1416) with respect to magnetic field sensor 1402. As target 1404moves, magnetic field sensor 1402 may detect changes in the reflectedfield to determine the position of target 1404 with respect to magneticfield sensor 1402. Of course, in other embodiments, target 1416 may bestationary and magnetic field sensor 1402 may move with respect totarget 1404.

As another example, multiple frequencies may be used to determine airgap and solve for position of the target 1404. For example, if thethickness of first material portion 1406 at end 1414 is greater than oneskin depth at a frequency f1, then the response at frequency f1 may varyonly as a function of air gap between target 1404 and the MR elements.Using a second frequency, if the thickness of first material portion1406 at end 1414 is less than one skin depth at a frequency f2, theresponse may vary as a function of both air gap and position of target1404.

Referring now to FIG. 14A, system 1400′ may include magnetic fieldsensor 1402 and a rotating target 1418, which may be in the shape of acylinder, a gear, etc. Target 1418 may include a first material portion1420 and a second material portion 1422. First material portion 1420 maybe a high-conductivity material, such as a metal; and second materialportion 1422 may be a relatively low-conductivity material, such as aplastic, ceramic, or other insulating material; or vice versa. Inembodiments, the first and second material portions 1420 and 1422 may bea unitary structure as may be integrally formed or may be separateelements physically coupled to each other, as shown in FIG. 14.

The thickness 1423 of first material portion 1420 may vary around thecircumference of target 1418 as a function of angle around target 1418so that, at point 1424, first material portion 1420 is relatively thinand, at point 1426, first material portion 1420 is relatively thick. Theeddy currents induced by magnetic field sensor 1402 in thicker portionsof first material 1420 may differ from those induced at thinnerportions. Accordingly, the reflected magnetic field produced at point1424 may also differ from the reflected magnetic field produced at point1426. Because the thickness of first material portion 1420 varies aroundthe circumference of target 1418 as a function of an angle around target1418, the reflected magnetic field may also vary around thecircumference.

Magnetic field sensor 1402 may be placed outside the radius of target1418, and adjacent to the outside surface of target 1418. Thus, themagnetic field sensing elements of magnetic field sensor 1402 may detectthe difference in the reflected magnetic field to determine therotational angle of target 1418. Magnetic field sensor 1402 may alsodetect rotational speed and/or direction of target 1418.

Referring now to FIG. 14B, system 1400″ may include magnetic fieldsensor 1402 and a rotating target 1428. Target 1428 may include a firstmaterial portion 1430 and a second material portion 1432. First materialportion 1430 may be a high-conductivity material, such as a metal; andsecond material portion 1432 may be a relatively low-conductivitymaterial, such as a plastic, ceramic, or other insulating material; orvice versa. In embodiments, the first and second material portions 1430and 1432 may be a unitary structure as may be integrally formed or maybe separate elements physically coupled to each other, as shown in FIG.14.

In FIG. 14B, the thickness of first material portion 1430 may extendinto the page. The thickness of first material portion 1430 may varyaround the circumference of target 1428 as a function of an angle aroundtarget 1428 so that, at point 1434, first material portion 1430 isrelatively thick and, at point 1436, first material portion 1430 isrelatively thin. The eddy currents induced by magnetic field sensor 1402in thicker portions of first material 1430 may differ from those inducedat thinner portions. Accordingly, the reflected magnetic field producedat point 1434 may also differ from the reflected magnetic field producedat point 1436. Because the thickness of first material portion 1430varies around the circumference of target 1428, the reflected magneticfield may also vary around the circumference.

Magnetic field sensor 1402 may be placed inside the radius of target1428, and adjacent to the substantially flat face 1440 of target 1428.In other words, if target 1428 is placed at the end of a rotating shaft,magnetic field sensor 1402 may be positioned adjacent to the face of oneend of the shaft. Thus, the magnetic field sensing elements of magneticfield sensor 1402 may detect the difference in the reflected magneticfield to determine the rotational angle of target 1428. Magnetic fieldsensor 1402 may also detect rotational speed and/or direction of target1418.

Magnetic sensor 1402 can be mounted in a gradiometer mode asillustrated, for example, in FIG. 3A. Half of the gradiometer may besituated at in a position where the distance between the conductive part1450 and the target remains substantially constant and half of thegradiometer may be situated in a position where the slope 1404 of theconductive material is present. The difference between the two signalsmay be used to suppress unwanted fluctuations due to the vibration ofthe target.

Referring to FIG. 15, system 1500 may include magnetic field sensingelement 1502 and target 1504. Magnetic field sensor 1502 may be the sameas or similar to magnetic field sensor 100 and/or any of the magneticfield sensors described above. Accordingly, magnetic field sensor 1502may include a coil to produce a magnetic field and produce eddy currentswithin target 1504, and one or more magnetic field sensing elements todetect a reflected field from the eddy currents.

Target 1504 may comprise a first material portion 1506 and a secondmaterial portion 1508. First material portion 1506 may be ahigh-conductivity material, such as a metal; and second material portion1508 may be a relatively low-conductivity material, such as a plastic,ceramic, or other insulating material; or vice versa. In embodiments,the first and second material portions 1506 and 1508 may be a unitarystructure as may be integrally formed or may be separate elementsphysically coupled to each other, as shown in FIG. 14.

First material portion 1506 may comprise a series of alternating wells1510 and valleys 1512. Wells 1510 may have a thickness 1514 relativelygreater than the thickness of valleys 1512. Accordingly, the reflectedmagnetic field produced within wells 1510 may differ from the reflectedmagnetic field produced at valleys 1512. Thus, the magnetic fieldsensing elements of magnetic field sensor 1502 may detect the differingmagnetic fields produced by wells 1510 and valleys 1512 as target 1504moves relative to magnetic field sensor 1502. The detected magneticfields may be used to detect speed, position, rotational angle, and/ordirection of magnetic target 1500, for example.

System 1500′ may include magnetic field sensor 1502 and target 1516.Target 1516 may comprise one or more first material portions 1518 and asecond material portion 1520. First material portions 1518 may be ahigh-conductivity material, such as a metal; and second material portion1522 may be a relatively low-conductivity material, such as a plastic,ceramic, or other insulating material; or vice versa.

First material portions 1518 may comprise a series of discrete wellspositioned in a spaced arrangement along the length of target 1516.Accordingly, when magnetic field sensor 1502 is adjacent to a tooth1518, a reflected magnetic field will be produced and detected. Whenmagnetic field sensing element is adjacent to an insulating area (e.g.area 1522), a reflected magnetic field may not be produced by theinsulating area 1522. Thus, the magnetic field sensing elements ofmagnetic field sensor 1502 may detect the reflected magnetic fieldsproduced by wells 1518 and detect when no reflected magnetic field isproduced as target 1516 moves relative to magnetic field sensor 1502.The detected magnetic fields may be used to detect speed and/ordirection of magnetic target 1516, for example.

Referring to FIG. 15A, system 1522 may include magnetic field sensor1502 and rotating target 1524. Target 1524 may comprise first materialportion 1526 and a second material portion 1528. First material portion1526 may be a high-conductivity material, such as a metal; and secondmaterial portion 1528 may be a relatively low-conductivity material,such as a plastic, ceramic, or other insulating material; or vice versa.

First material portions 1526 may comprise one or more teeth 1530positioned in a spaced arrangement around the circumference of target1524 at various angles around target 1524. Although two teeth are shown,target 1524 may include one tooth, two teeth, or more teeth in spacedrelation around the circumference of target 1524. The teeth may bespaced evenly or in an uneven pattern.

Accordingly, when magnetic field sensor 1502 is adjacent to a tooth1530, a reflected magnetic field will be produced and detected. Whenmagnetic field sensing element is not adjacent to tooth, a reflectedmagnetic field with a different strength may be produced by firstmaterial portion 1526. Thus, the magnetic field sensing elements ofmagnetic field sensor 1502 may detect the reflected magnetic fieldsproduced by teeth 1530, and the reflected magnetic field produced byareas of first material 1526 without teeth, as target 1524 rotatesrelative to magnetic field sensor 1502. The detected magnetic fields maybe used to detect speed and/or direction of magnetic target 1500, forexample.

Referring to FIG. 15B, system 1522′ may include magnetic field sensor1502 and rotational 1532. Target 1532 may comprise one or more firstmaterial portions 1534 and a second material portion 1536. Firstmaterial portions 1534 may be a high-conductivity material, such as ametal; and second material portion 1536 may be a relativelylow-conductivity material, such as a plastic, ceramic, or otherinsulating material; or vice versa.

First material portions 1534 may comprise a series of discrete wellspositioned in a spaced arrangement around a radial circumference oftarget 1532. First material portions 1530 may be spaced evenly, oraccording to any type of pattern. Accordingly, when magnetic fieldsensor 1502 is adjacent to one of the first material portions 1534, areflected magnetic field will be produced and detected. When magneticfield sensor 1502 is adjacent to an insulating area (e.g. area 1538), areflected magnetic field may not be produced by the insulating area1538. Thus, the magnetic field sensing elements of magnetic field sensor1502 may detect the reflected magnetic fields produced by first materialportions 1534 and detect when no reflected magnetic field is produced byinsulating areas 1538 as target 1532 rotates relative to magnetic fieldsensor 1502. The detected magnetic fields may be used to detectrotational speed and/or direction of magnetic target 1532, for example.

Magnetic field sensor 1502 may be placed inside the outermost radius oftarget 1532, and adjacent to a substantially flat face 1540 of target1532. In other words, if target 1532 is placed at the end of a rotatingshaft, magnetic field sensor 1502 may be positioned adjacent to the faceof one end of the shaft. Thus, as target 1532 rotates, the magneticfield sensing elements of magnetic field sensor 1502 may detect firstmaterial portions 1534 as they pass by.

Referring to FIG. 15C, system 1522″ may include magnetic field sensor1502 and rotational target 1532. Target 1532 may comprise one or morefirst material portions 1534′ and a second material portion 1536. Firstmaterial portions 1534 may be a high-conductivity material, such as ametal; and second material portion 1536 may be a relativelylow-conductivity material, such as a plastic, ceramic, or otherinsulating material; or vice versa.

First material portions 1534′ may comprise several series of discretewells positioned in a spaced arrangement around different radialcircumference of target 1532. First material portions 1530 may be spacedevenly, or according to any type of pattern. Accordingly, when magneticfield sensor 1502 is adjacent to one of the first material portions1534, a reflected magnetic field will be produced and detected. Whenmagnetic field sensor 1502 is adjacent to an insulating area (e.g. area1538), a reflected magnetic field may not be produced by the insulatingarea 1538. Thus, the magnetic field sensing elements of magnetic fieldsensor 1502 may detect the reflected magnetic fields produced by firstmaterial portions 1534 and detect when no reflected magnetic field isproduced by insulating areas 1538 as target 1532 rotates relative tomagnetic field sensor 1502. The second radial series of wells may bearranged so that each well 1560 in the second radial series is placedadjacent to a gap 1562 between the wells 1534 in the first radialseries. As magnetic field sensor 1502 detects each radial series, theremay be a 90-degree shift of phase or a different pitch between detectionof the first radial series of wells and the second radial series ofwells, which may be used to increase the accuracy of angle by a Verniertype of approach.

Magnetic field sensor 1502 may be placed inside the outermost radius oftarget 1532, and adjacent to a substantially flat face 1540 of target1532. In other words, if target 1532 is placed at the end of a rotatingshaft, magnetic field sensor 1502 may be positioned adjacent to the faceof one end of the shaft. Thus, as target 1532 rotates, the magneticfield sensing elements of magnetic field sensor 1502 may detect firstmaterial portions 1534 as they pass by.

Referring to FIG. 16, system 1600 may include a first magnetic fieldsensor 1602, a second magnetic field sensor 1604, and a rotating target1606. Magnetic field sensors 1602 and 1604 may be the same as or similarto magnetic field sensor 100 and/or any of the magnetic field sensorsdescribed above.

Target 1606 may include a spiral inclined plane 1608 positioned around acentral axis 1610. In embodiments, central axis 1610 may be a rotatingshaft. Target 1606 may also include a conductive reference portion 1612.Reference portion 1612 and inclined plane 1608 may be formed fromconductive material.

In an embodiment, magnetic field sensor 1602 is positioned adjacent toreference portion 1612. A coil of magnetic field sensor 1602 produces amagnetic field, which in turn produces eddy currents in referenceportion 1612. Magnetic field sensor 1602 may detect the reflectedmagnetic field produced by the eddy currents.

Similarly, magnetic field sensor 1604 may be positioned relative toinclined plane 1608. A coil of magnetic field sensor 1608 may produce amagnetic field, which in turn may produce eddy currents in a portion1614 of inclined plane adjacent to magnetic field sensor 1604. Magneticfield sensor 1604 may detect the reflected magnetic field produced bythe eddy currents in inclined plane 1608.

As target 1606 rotates, the portion 1614 of inclined plane 1608 adjacentto magnetic field sensor 1604 will move toward and/or away from magneticfield sensor 1604. The proximity D of portion 1614 to magnetic fieldsensor 1604 can be detected by magnetic field sensor 1604. Processingcircuitry (not shown) can correlate the proximity D to a rotationalangle of target 1606 and determine position, speed of rotation,direction of rotation, etc.

Referring to FIG. 16A, system 1600′ may include a grid of magnetic fieldsensors 1616, and a rotating target 1606.

Target 1606 may include a spiral inclined plane 1608 positioned around acentral axis 1610. In embodiments, central axis 1610 may be a rotatingshaft. Target 1606 may also include a conductive reference portion 1612.Reference portion 1612 and inclined plane 1608 may be formed fromconductive material.

In an embodiment, magnetic field sensor 1602 of grid 1616 is positionedadjacent to reference portion 1612. A coil of magnetic field sensor 1602produces a magnetic field, which in turn produces eddy currents inreference portion 1612. Magnetic field sensor 1602 may detect thereflected magnetic field produced by the eddy currents.

The other magnetic field sensors 1618 a-h may be positioned in variouslocations on the grid 1616 relative to inclined plane 1608. A coil ofeach of magnetic field sensors 1618 a-h may produce a magnetic field,which in turn may produce eddy currents in a portion of the inclinedplane adjacent to each magnetic field sensor 1618 a-h, which may eachdetect the local reflected magnetic field produced by the eddy currentsin inclined plane 1608.

As target 1606 rotates, the portions of inclined plane 1608 adjacent tomagnetic field sensors 1618 a-h will move toward and/or away frommagnetic field sensors 1618 a-h. The proximity D of any portion 1614 toany magnetic field sensor 1618 a-h can be detected by each magneticfield sensor. Processing circuitry (not shown) can correlate theproximity D to a rotational angle of target 1606 and determine position,speed of rotation, direction of rotation, etc.

Referring to FIG. 16A, a plurality of sensors 1618 a-h forming a gridmay be used to measure the distance of the spiral at different points soit allows to correct vibrations of the spiral on directionsperpendicular to the axe of rotation whereas the central sensor of thegrid is suppressing the vibrations along the axe of rotation.

Referring to FIG. 17, a substrate 1700 may support one or more of themagnetic field sensor circuits described above, including coils andmagnetic field sensing elements. Substrate 1700 may be positioned (andadhered to) frame 1702. Substrate 1700 may be a semiconductor substrate,a glass substrate, a ceramic substrate, or the like. Bond wires 1704 mayelectrically couple connection pads on substrate 1700 to leads of frame1702. Frame 1702 may be a lead frame, a pad frame, or any structure thatcan support substrate 1700.

In embodiments, substrate 1700 may support coil 1701, which may be thesame as or similar to the coils described above. Coil 1701 may produce amagnetic field that may induce eddy current and a reflected magneticfield in a target and/or a magnetic field that may be directly coupledto (e.g. directly detected by) MR elements. As shown, coil 1701 may bepositioned adjacent to (or opposite) a gap 1703 in frame 1702. If frame1702 is a conductive material (such as metal), the magnetic fieldproduced by coil 1701 could induce eddy currents and a reflected fieldfrom frame 1702. Placing coil 1701 near gap 1703 may reduce or eliminateany unwanted reflected field that might otherwise by generated by frame1702.

In FIG. 17A, substrate 1706 may support one or more of the magneticfield sensor circuits described above, including coils and magneticfield sensing elements. Substrate 1706 may be positioned (and adheredto) lead frame 1707. Substrate 1706 may include one or more vias 1708,which may be coupled to solder balls (or solder bumps) 1710. Solderballs 1710 may be coupled to leads of lead frame 1707 to provide anelectrical connection between vias 1708 and leads of lead frame 1707.The electrical connection may couple the sensor circuitry (generallysupported by one surface of substrate 1700) to external system andcomponents through leads 1707.

In embodiments, substrate 1706 may support coil 1709, which may be thesame as or similar to the coils described above. Coil 1709 may produce amagnetic field that may induce eddy current and a reflected magneticfield in a target and/or a magnetic field that may be directly coupledto (e.g. directly detected by) MR elements. As shown, coil 1709 may bepositioned adjacent to (or opposite) a gap 1705 in frame 1707. If frame1707 is a conductive material (such as metal), the magnetic fieldproduced by coil 1709 could induce eddy currents and a reflected fieldfrom frame 1707. Placing coil 1709 near gap 1705 may reduce or eliminateany unwanted reflected field that might otherwise by generated by frame1707.

In embodiments, the grid of sensors 1608 a-h in FIG. 16A may be formedon the surface of substrate 1700 or 1706.

Referring to FIG. 18, a magnetic field sensor circuit 1800 may besupported by one or more substrates. As shown in FIG. 18, a firstsubstrate 1802 may support one or more coils 1804, 1806, which mayproduce a magnetic field. A second substrate 1808 may support one ormore magnetic field sensing elements 1810, which may detect thereflected magnetic field as discussed above. The semiconductor dies1802, 1808 may also include additional circuitry discussed above.Circuits supported by substrate 1802 may be electrically coupled tocircuits supported by substrate 1808 with lead wires (not shown). Thesupported circuits may also be coupled to leads of a frame 1811 by leadwires. A semiconductor package (not shown) may enclose the substrates.

In an embodiment, second die 1808 may be glued to a top surface of firstdie 1802. Alternatively, die 1808 may be reversed and electricallyconnected to die 1802 with die-to-die electrical connections.

The magnetic fields produced by coils 1804 and 1808 may cancel eachother out in the area between coils 1804 and 1806, i.e. the area whereMR elements 1810 are positioned. Thus, substrate 1808 may be positionedso that MR elements 1810 fall within the area where the magnetic fieldscancel, to minimize any stray or directly coupled field detected by MRelements 1810.

In embodiments, substrates 1802 and 1808 may be different types ofsubstrates. For example, substrate 1802 may be an inexpensive substratefor supporting metal traces such as coils 1804 and 1806, while substrate1808 may be a substrate for supporting MR elements and/or otherintegrated circuits.

Referring to FIG. 18A, a magnetic field sensor circuit 1800′ may besupported by multiple semiconductor dies. As shown, a first die 1812 maysupport two (or more) sets of coils. A first set of coils may includecoils 1814 and 1816. A second set may include coils 1818 and 1820. Asecond die 1822 may support a first set of magnetic field sensingelements 1824, and a third die 1826 may support a second set of magneticfield sensing elements 1828.

In an embodiment, magnetic field sensor circuit 1800′ may include twomagnetic field sensors. The first sensor may include coils 1814 and1816, die 1822, and magnetic field sensing elements 1824. The secondmagnetic field sensor may include coils 1818 and 1820, die 1826, andmagnetic field sensing elements 1828. In other embodiments, magneticfield sensor circuit 1800′ may include additional magnetic field sensorscomprising additional coils, dies, and magnetic field sensing elements.

Magnetic field sensor circuit 1800′ may be used in any of the systemsdescribed above that employ two (or more) magnetic field sensors.Additionally or alternatively, the two magnetic field sensors in circuit1800′ may be driven at different frequencies to avoid cross-talk betweenthe two sensors.

Referring to FIG. 19, a magnetic field sensor circuit 1900 may besupported by multiple substrates. A first substrate may support coil1902. Four smaller substrates 1904-1910 may each support one or moremagnetic field sensing elements. As shown, substrates 1904-1910 may bepositioned adjacent to traces of coil 1902. In some embodiments,substrates 1904-1910 may be positioned so the magnetic field sensingelements they support are placed adjacent to gap 1912 between traces ofcoil 1902.

A fifth substrate 1914 may support circuitry to drive coil 1902 and themagnetic field sensing elements, as well as processing circuitry toprocess signals received from the magnetic field sensing elements.Circuits on the various die may be coupled together by lead wires 1916.

Although not shown, in another embodiment, the larger substrate 1402 maysupport the coils and MR elements. The smaller substrate 1904-1908 maysupport circuitry to drive the coils and MR elements and/or circuits toprocess the magnetic field signals.

In an embodiment, the magnetic field sensing elements and coil 1902 maybe the same as or similar to the magnetic field sensing elements (e.g.MR elements) and coils described in some or all of the magnetic fielddetection systems described above.

Having described preferred embodiments, which serve to illustratevarious concepts, structures and techniques, which are the subject ofthis patent, it will now become apparent to those of ordinary skill inthe art that other embodiments incorporating these concepts, structuresand techniques may be used. Accordingly, it is submitted that that scopeof the patent should not be limited to the described embodiments butrather should be limited only by the spirit and scope of the followingclaims. All references cited herein are hereby incorporated herein byreference in their entirety.

1. A magnetic field sensor comprising: at least one coil responsive toan AC coil drive signal; at least two spaced apart magnetic fieldsensing elements responsive to a sensing element drive signal andpositioned proximate to the at least one coil; and a circuit coupled tothe at least two magnetic field sensing elements to generate an outputsignal of the magnetic field sensor indicative of a difference between adistance of a conductive target with respect to each of the at least twospaced apart magnetic field sensing elements.
 2. The magnetic fieldsensor of claim 1, wherein at least one of the magnetic field sensingelements is configured to detect a directly coupled magnetic fieldgenerated by the at least one coil and a reflected magnetic fieldreflected by the conductive target and wherein a configuration of the atleast one coil and the at least two magnetic field sensing elements isselected in order to achieve a predetermined level for the directlycoupled magnetic field.
 3. The magnetic field sensor of claim 2, whereinthe configuration of the at least one coil and the at least two magneticfield sensing elements is selected to minimize the predetermined levelof the directly coupled magnetic field.
 4. The magnetic field sensor ofclaim 3, wherein the at least one coil comprises a pair of coils andwherein the at least two magnetic field sensing elements are disposedbetween the coils.
 5. The magnetic field sensor of claim 1, wherein theat least one coil comprises a single coil.
 6. The magnetic field sensorof claim 1, wherein the at least one coil comprises a first portionconfigured to have a current flow in a first direction and a secondportion configured to have a current flow in a second direction andwherein at least one of the magnetic field sensing elements is disposedabove the first portion of the at least one coil and at least one of themagnetic field sensing elements is disposed above the second portion ofthe at least one coil.
 7. The magnetic field sensor of claim 1, whereinat least one of the magnetic field sensing elements is aligned with agap of the at least one coil.
 8. The magnetic field sensor of claim 1,wherein the at least one coil comprises a first plurality of loops and asecond plurality of loops spaced from the first plurality of loops by agap and wherein at least one of the magnetic field sensing elements isaligned with the gap.
 9. The magnetic field sensor of claim 1, whereinthe at least one coil comprises at least two layers and wherein at leastone of the magnetic field sensing elements is disposed between the atleast two layers of the at least one coil.
 10. The magnetic field sensorof claim 1, wherein the at least two magnetic field sensing elementscomprise at least four magnetic field sensing elements electricallycoupled to form at least one bridge.
 11. The magnetic field sensor ofclaim 1, wherein the at least two magnetic field sensing elementscomprise at least eight magnetic field sensing elements electricallycoupled to form at least two bridges.
 12. The magnetic field sensor ofclaim 1, wherein the conductive target comprises a pressure susceptibleelement and the output signal is further indicative of a pressureassociated with the pressure susceptible element.
 13. The magnetic fieldsensor of claim 1, wherein the AC coil drive signal has a firstfrequency and the sensing element drive signal has a second frequency.14. The magnetic field sensor of claim 13, wherein the first frequencyof the AC coil drive signal is selected based at least in part on a skineffect associated with the conductive target.
 15. The magnetic fieldsensor of claim 13, wherein the AC coil drive signal and the sensingelement drive signal are provided by a common source and wherein thefirst frequency of the AC coil drive signal and the second frequency ofthe sensing element drive signal are substantially the same.
 16. Themagnetic field sensor of claim 15, wherein the first frequency andsecond frequency are selected in response to changing the firstfrequency from a first frequency value to a second frequency value andcomparing the output signal associated with the first frequency value tothe output signal associated with the second frequency value to detect adisturbance.
 17. The magnetic field sensor of claim 13, wherein the ACcoil drive signal and the sensing element drive signal are provided byindependent sources.
 18. The magnetic field sensor of claim 17, whereinthe second frequency of the sensing element drive signal isapproximately DC.
 19. The magnetic field sensor of claim 17, wherein thefirst frequency of the AC coil drive signal is selected in response tochanging of the first frequency from a first frequency value to a secondfrequency value and comparing the output signal associated with thefirst frequency value to the output signal associated with the secondfrequency value to detect a disturbance.
 20. The magnetic field sensorof claim 1, wherein the circuit comprises a low pass filter.
 21. Themagnetic field sensor of claim 1, wherein the circuit comprises atemperature compensator responsive to a temperature sensor and to amaterial type selector.
 22. The magnetic field sensor of claim 1,wherein the circuit comprises a linearization module.
 23. The magneticfield sensor of claim 1, wherein the AC coil drive signal has a firstphase, wherein each of the at least two magnetic field sensing elementsgenerates a respective magnetic field signal, and wherein the magneticfield sensor further comprises at least one feedback coil driven by asecond AC coil drive signal having a second phase and disposed adjacentto the at least two magnetic field sensing elements, wherein the circuitis configured to adjust the second AC coil drive signal in order toachieve a predetermined level for the magnetic field signals generatedby the at least two magnetic field sensing elements.
 24. The magneticfield sensor of claim 23, further comprising a current detection circuitto detect the second AC coil drive signal and to provide the outputsignal of the magnetic field sensor based on the detected second AC coildrive signal.
 25. The magnetic field sensor of claim 1, wherein the ACcoil drive signal has a first phase, at least one of the magnetic fieldsensing elements detects a reflected magnetic field reflected by theconductive target and having a second phase, and wherein the circuitcomprises a demodulator responsive to a difference between the firstphase and the second phase to demodulate the magnetic field signal. 26.The magnetic field sensor of claim 1, wherein the at least two magneticfield sensing elements comprise one or more of a Hall effect element, agiant magnetoresistance (MR) element, an anisotropic magnetoresistance(AMR) element, a tunneling magnetoresistance (TMR) element, or amagnetic tunnel junction (MTJ) element.
 27. A magnetic field sensorcomprising: a substrate; at least one coil supported by the substrateand responsive to an AC coil drive signal; a first set of at least twomagnetic field sensing elements disposed at a first position withrespect to the coil; a second set of at least two magnetic field sensingelements disposed at a second position with respect to the coil, thesecond position spaced from the first position, wherein the second setof magnetic field sensing elements is electrically coupled to the firstset of magnetic field sensing elements to form a bridge; and a circuitcoupled to the bridge to generate an output signal indicative of adifference between a first distance of a conductive target with respectto the first set of magnetic field sensing elements and a seconddistance of the conductive target with respect to the second set ofmagnetic field sensing elements.
 28. A magnetic field sensor comprising:at least one coil responsive to an AC coil drive signal; at least twospaced apart magnetic field sensing elements responsive to a sensingelement drive signal and positioned proximate to the at least one coil;and means for generating an output signal of the magnetic field sensorindicative of a difference between a distance of a conductive targetwith respect to each of the at least two spaced apart magnetic fieldsensing elements.